You'll need a few things:
If you can possibly afford the plaster bandages, do that. The problem with plaster is that it gives off heat as it sets, and the thicker you make it, the faster it gets hot, so it gets up to a higer temperature. If you're doing DIY plaster "bandages", it's not too hard to overdo the thickness, and really make your model uncomfortable. I've done this exactly once, to the point where her skin got pretty pink, and never again.
Decide beforehand how much of yourself you want to cast. Fine hairs won't disturb anything, and won't be pulled or lost in the alginate. Coarse, or curly, or closely-spaced hair must either be removed, or covered with a release agent. Vaseline works, but I find it leaves a not-quite-natural impression in the mold. Cholesterol-based hair conditioner is thick enough to fill in and lubricate the hair, but not oily and so doesn't repel the alginate. If you're shy about your pubic region, cover yourself with your hand and arm, and just put that in the casting. If you do use your hand, keep very good contact between your arm and your body; thin layers of alginate tend to tear easily. Too much area is better than too little; you can always use woodworking tools to trim and finish the edges of the casting later. Wear crummy clothes, if any, or get a polyester barber's cape to wrap over clothes. The alginate does run down and sticks to cloth. If you wait until it's well set to peel it off, you usually can, though.
Since we're talking about pregnancies, you may want to get your breasts in there too, to get the whole fertility-symbol thing going. If you do, you'll want to be sitting up. If you make the mold lying down, your breasts will hang to the sides, not down, and that looks really odd when you put the finished piece up on the wall. When I do a commission, I usually haver the woman sit leaning a little bit back against some cushions. That lets the model rest, but gives a natural look to the breasts. If you're also wanting to get your groin in, then figure out some way to kneel and lean back slightly. DO NOT try to stand up in the exact same position for the half-hour or 45 minutes this is going to take. It's no fun, and unless you're a pro art model, there is some risk of passing out just from holding the pose. In cases like this, I use a stepladder. It gives your shins somewhere to go, and there are steps to sit on, plus rails to grab for stability.
Prep. Put down lots of newspaper. Get everything ready beforehand, because once the alginate is mixed, you've got 5-7 minutes, period. Cut pieces of batting a bit bigger than you think you'll need. Pull it apart; it comes folded together. First, completely unfold it, and then pull it apart so you've got two halves, each with a kind-of-smooth side, and a rough, fuzzy side. This goes for both the cotton and the polyester batting.
Mix the alginate. I use a mix BY WEIGHT of 2 parts alginate to 7 parts water. This is pretty stiff, but will slump a little bit on its own. For doing a large piece, like a torso, two pounds of alginate and seven pounds (pints) of water is about right. Make a few small test batches first to see how the consistency works.
The water should feel just comfortably warm. You want it warm enough to not raise goose bumps, but not much warmer, or the alginate will set up a bit too fast.
Mixing should take about a minute. Put the water in the bucket first, and then the alginate powder, and start mixing right away. There'll be a lot of dust at first, but just keep working the mixer on the drill up and down, to move the thinner, wetter mix at the bottom up to grab more powder. Completely contrarily, do your best not to mix in air. The less air you get in the mold, the less re-work you'll do later. I solve this problem by using a vacuum pump to remove the air; you're on your own, unfortunately. Don't mix until every single little last lump is out, but do mix until it's nice and smooth and just a bit thicker than molasses. It should slump and move under its own weight, but not too much.
Once it's mixed, take a handful at a time, and smoosh it onto the model. Press pretty firmly. You want to get the alginate into good contact all over the skin, to get rid of air bubbles and pockets. Go over the whole surface once, thinly, just to get everything covered. Next, go back over. Work from top to bottom, letting gravity pull the alginate down for you. The second layer should ideally be about 1/4 inch thick (at least) everywhere. If you're doing breasts or other overhanging things, spend a few extra seconds to make sure there are no air bubbles caught under the breast. This is really common.
Once you've got the second layer on, it's time to grab the batting. Press it firmly into the surface as you apply it, fluffy side down. What you're doing is putting some fibers into the alginate, but sticking out so that plaster can grab them. The alginate itself will stick to nearly nothing once it's started to set. This is really really important. When the alginate begins to set, it will get a little grainy, rather than completely smooth. If you haven't already started getting the batting pressed in by this point, work *really* fast. Having two people can make things easier here.
Next, mix up a small batch of plaster, maybe a quart or a quart and a half. How much water to use depends on the type of plaster. It can vary anywhere from 4 parts plaster to 1 part water (by weight) up to 3 plaster to 2 water. Read the package directions. Before you start putting the plaster on, gently pull on the batting. If you used cotton, most of the back layer will come off, leaving a nice thin layer of fibers stuck in the alginate. If you used the poly batting, same deal, mostly. If you put in the poly really early, nearly all of it will sink in, and there's nothing you need to pull off.
Once the plaster mixed, spread this thinly over the alginate and remaining fibers. This layer is not supposed to give any strength, it's just to fill in any low points. The reason for the plaster layer is to make a mostly rounded surface for the plaster bandages in the next step. If you roll on the bandage, and it stretches over a low spot, then there'll be a gap there. "Fine", you think. Until you turn the mold over, and pour all that heavy plaster in. The alginate, being rubbery, will sink down into that gap. Where the alginate moved, there'll be a bump on your cast. Basically, just keep working the plaster until it thickens up enough to stay where you put it, then fill in gaps and low spots. Specifically for a pregnancy, I fill in under the breasts until they're evenly domed, and a bit between the breasts, to level the valley a bit.
As mentioned earlier, watch out for the plaster heating up. If you use a thin layer most places, you shouldn't have a problem. Remember, this layer is not for strength or solidity, just to even and level the surface.
Once the plaster has started to set, you can put on the plaster bandage. Get a good, sharp pair of scissors. Fold the end of the bandage around one finger, so you can find it later. Dunk the whole roll into cold water for 5 or 7 seconds, until it mostly stops bubbling out air. Squeeze a bit of water out gently, just enough so that it's not really drippy. Pull off a length, and scissor it off. Lay the bandage on the plaster, and smooth it out with your fingers to spread the plaster around. Keep going, pressing the bandage into corners, and generally making a good fit to the alginate. Overlap the bandages by about half of their width. Do at least four layers in different directions. Me, I'm paranoid. At this point, I've put a lot of work into the piece, so I usually go with 8 layers. For a female torso, that usually takes four or five bandages. I use bandages that are six inches wide, and five yards long.
If you're using homemade "bandages", mix up some plaster with accelerator. Smear the plaster over the alginate/cotton, then lay in another layer of cotton, then more plaster, etc. Three or four layers is probably enough. If you see it cracking or breaking when you try to remove the mold, just ease the mold back, and layer on more. BE CAREFUL. Too much plaster all in one lump sets up faster, which gives off heat. The hotter plaster gets, the faster it sets, and the more heat it releases. You can probably see where this is going. If you're really incautious, and your model is too stoic for her own good, you can make her very uncomfortable and unhappy, at best. I really strongly recommend using the plaster bandage, not DIY. It's better, it's stronger, it's easier to pull off when you un-mold your cast. Do this only if you can not afford the proper bandages.
You can tell the bandages have set when you tap on them, and you hear a sharp "tap" sound, not "thud", or "squish". Wait a few minutes more to be sure they're good and strong.
Now, you're ready to get the mold off. The alginate is still pretty soft, so you need to be as careful as you can. Work slowly. Get a little bit of skin loose at a time. It's really easy to feel where it's already loose, and where it's still sticking to you. Get the edges loose first. You'll be able to feel the mold coming loose as it peels away.
If the model is sitting up, it's usually pretty straightforward to tip the mold away from her at the top, and just keep going until the alginate is cradled in the plaster. If lying down, then try to pick it up as little as possible, and just sort of roll it over to the side. If you did a good job with getting fibers into the alginate, you don't need to worry too much about this step. If there weren't many fibers, be really careful. The alginate will slip around, and may tear.
Once it's off, you want to mix and cast the plaster as soon as you can. The alginate is continuing to set, and changing shape subtly. Don't wait even as much as overnight. Mix the plaster according to the directions on the package. Do not use accelerator for the plaster for your casting. For a torso sized piece, I use enough plaster and water to get 1.5 gallons of mix. If you're using TufStone, that'll take 18 lbs of plaster, and 3 quarts of water.
You might also want to use latex gloves, or dishwashing gloves for putting the plaster in your mold. In addition to not having the plaster dry out your skin, I find that if I'm even a little careless, my fingernails nick the surface of the alginate.
Pour a bit of plaster into the mold, and slosh it around, so it coats the whole surface. Be sure to cover the whole surface first. If you don't, you'll get little gaps where a later batch didn't quite run up to the first. Blow on, or gently touch bubbles to break them. As the plaster starts to set up, take a bit more at a time from the bucket and apply it into the mold. Eventually, the plaster will get to a sort of clay-like consistency. This won't last long, so be constantly lifting and smoothing the plaster up the sides of the mold. As soon as it will stay on its own, get it where you want it, then leave it alone. Over-working it once it has gotten firm will just crack it.
If you're going to hang the piece on a wall, get several short lengths of reasonably good rope. No, not clothesline. You want maybe a foot to ten inches for each piece. Tie a simple knot at each end of the rope. Bend the rope into a U shape, and lay it on the back of the plaster. Scoop up some more of the clay-ey plaster, and press it down over the ends of the rope to secure it.
Leave a piece that big to set for at least a couple of hours. Overnight is better, if you can stand to wait. De-molding is probably just a matter of turning the whole deal over, then *carefully* lifting the alginate mold off, again, going slowly. From a mold this size and shape, you're only going to get one good casting. If you made your own bandages, they may be too stiff to flex away, and you may need to carefully cut through the plaster to get smaller sections that will pull away. If you do, use the blue of the alginate to tell when you're through the outer plaster.
How you finish the piece depends on your taste. I really like to preserve the texture of the skin; with this alginate, you can clearly see individual pores. If you want to preserve this texture, DO NOT TOUCH THE CASTING once you have un-molded it. Wait at least a day days until it is bone dry to the touch.
If you're inclined to smooth edges, use a fine-toothed saw (I use a hacksaw blade) to trim, followed by a rasp. You will certainly have some surface blobs. If small, these can usually just be popped off. For cleaning up minor flaws, I use several different dental tools. To fill little bubbles or gaps in the plaster, just mix up more plaster and water, and press it in with a finger or a tool. Be a bit careful: smearing too much around will fill in detail that you might want to save. Also, be sure to get the mix ratio pretty close to what you originally used - too thick or too thin will be a different color, sometimes noticeably so.
If you just want the white finish (shows shadows and fine details quite well), I recommend leaving the plaster surface just as it is. With a gypsum cement, the surface is hard enough to be well-protected without any further effort. You can clear-coat the piece with spray lacquer, but be aware that you'll lose detail, just like water on frosted glass.
Another option is to sand the surface smooth (be very very thorough; use 100, then 200, then 400 grit sandpaper, and get every last flaw out), and then use a product called Rub-'n-Buf. It's in craft stores near the gold-leafing and oil paint stuff. It comes in a little tube, and smells suspiciously like paste shoe polish. It comes in several colors, but it's basically brass or aluminum powder, and possibly pigment (depending on color). It does an amazingly good job of adding a gold-leaf look to things with basically none of the hassle. (Yes, I tried using real leaf, but on surfaces as uneven as faces, it's a lot more work than it's worth) Again, this option really does a good job of preserving surface detail, but I find the result to be more pleasing in appearance if it's sanded smooth first.
For painting, you're pretty much on your own. I will suggest only that you cast up some of the same plaster you used for the piece to practice on. I've tried paint a little bit, and the plaster is so porous and absorbent that you definitely need to prime it first.