Layout strips of plaster gauze, have a variety of sizes. Most should be between 1 - 4 inches in width and from 6 - 14 inches in length. Not many of the 1 inch size is used.
Cover clothing with a thin layer of plastic wrap. It should be as form fitting as possible, but not tight to make it hard to breath, or distort the body shape.
Make a tape ridges and tape it vertically along the center front and tape it to the skin down the front and back from the neck down to the about where the inseam starts. This is mainly used to keep the left and right sides of the mold in a somewhat orderly fashion, and also to keep them from sticking to each other.
Cover the upper torso of the subject with a generous layer of Vaseline.
Have a bowl of warm water ready. Wearing gloves, start to cover one side from the shoulders down. Work quickly as the plaster sets up quickly and adhesion is best when the prior section is still wet. Use at least 2 to 3 layers to insure the mold has sufficient integrity. Work in alignment blocks on to the front and back of the mold along the split center line. Also add in 3 blocks on each side upper shoulder, under arm, and hip. It works best if they are parallel to each other and run straight front to back. If they don't sit right, build up underneath as needed with additional gauze. Run at least two double layers over each side of each block, and do a cross wrap above and below each in between. This helps them to stay in place as they don't stick perfectly to the plaster.
Allow the plaster to harden and set fully. It should be past the point where it is very hard. Carefully remove the tape if needed and then slide the mold off to the side. Repeat with the other side.
Clean the inside of the mold with a damp cloth to remove any Vaseline residue. Try not to get it too wet.
Mix up small amounts of plaster of Paris, and smooth over any ridges on the inside of each mold half. Save anything near the outside edges for joining. Smooth over with a squeegee or a gloved hand. Any bumps added will need to be sanded off, so try to be smooth and quick.
Use the alignment pins to join the halves together. Use a few layers of gauze on the outside of the front and back seams. Cover over the armholes with a few layers as well.
Mix up plaster of Paris and fill in and smooth over any gaps on or near the front and back seams and areas nearby that were not previously done.
Allow to dry thoroughly, overnight works well.
Remove the bolts from the side alignment blocks. Use a saw to cut the haves into front / back molds going between the alignment blocks. Remove any raised blobs if present from the inside of the mold.
Cover the mold with a sealer. Kills 2 latex paint was used for the first round, Shellac will be used next.
Once dry, cover the mold with a release agent. The first test used Vaseline, the second attempt will use a layer of paste wax.
Place a 1 inch or larger dowel from the top to bottom of the mold so it pops out of the neck on top. Make a cardboard cover that covers the remainder of the neck opening. cover it with plastic wrap and also coat the inside with release agent.
wrap the outside of the mold with a number of layers of plastic wrap. This will help to hold the mold together during the foam expansion, and minimize leakage.
Mix equal parts of polyurethane foam. It is a two part mixture mixed 1:1. Use two individual cups to measure out small batches. Pour both into a fresh mix cup. Stir quickly and thoroughly to insure it is mixed well. You'll have less than a minute. As the foam starts to warm up you'll see bubbles starting to build. Pour the foam into the mold. Repeat this process until full. The foam will expand for about 4 minutes. Try to time it so additional layers are added after the prior layer has fully risen.
Allow to fully cure. It doesn't really take that long, but wet sticky foam is nasty, so you might as well wait until the next day.
Remove the mold. If the release agent works well, then it should pop out. If it sticks and has to be torn off a little bit at at time over the course of a few hours, then it will be working like our first attempt. A wood mallet worked to chip off some of the more stubborn chunks. Clean off the dressform to remove any release agent. This seemed nearly pointless after the effort to extract it had more or less removed everything already. Any high spots or ridges can be smoothed over with sandpaper if needed. A random orbital sander is also helpful for removing bits of stubborn mold residue and also smoothing over any high spots that shouldn't be there.
Mount on stand or hang from top.
How to make a dummy 
How to make a dress form cover: