Here are directions from someone named "JOAN": I'm sorry I don't know whom to credit this information to, but it's quite useful!
"The Following is a list of directions that correspond to how I actually went about constructing a plaster casted dress form. I am not the original poster of this info. I would not follow these directions like a bible and would do what works for you. I certainly do not want to be responsible for you getting stuck...
The materials are... plaster casting strips that are available at craft shops. I used 7-8 rolls... but I am a very little person and the dress form was not to be used as it was intended. It is now a display piece so not much care was taken in getting the shapes correct, so it might take more or less plaster strips. Plastic wrap or dry cleaning bag is a must. You might also want to use rib knit over the plastic for strength. Foam insulation in a can for filling the casting or something simular. I also made a stand of a pine 2x2 and wood shelving supports. I also used a wooden drapery ball for the top of the form which was attached at the neck through to the 2x2.
The process is as follows: One must be dressed in the appropriate clothing items. I as in a bra and panties. My dh then wrapped me in plastic wrap. Don't do it too tight, you are going to be uncomfortable when the plaster goes on so don't make it worse by wrapping the plastic too tight. He then pulled a tube of knit ribbing over the plastic. We made slits at the top to fit around my arms. We then taped the top up and over the shoulders. Now comes the plaster strips, we started at the top but I think that it would have been better to start at the bottom. I didn't want one that went below the hip but I recommend starting below the hip so that you can get full benefit of the finished dress form. The plaster strips should not be too long or they are too hard to handle. The strips should conform to your figure and can be pressed into shape. Take your time, it will pay off in the end. We did the strips about 2-4 layers thick. Work your way up and try to get it as even as you can around the bottom, neck and arms, this will pay off later. Try to get it as smooth as possible. Now comes the wait. I think that it took about 30 minutes. Try not to move too much. You want your shape to freeze into the plaster. I didn't have too much trouble except that my dh kept on laughing at me and threatening to leave me in the plaster. You don't want to wait until it is totally dry. You will know when it is time... I hope. We cut a seam in the side from the bottom to one armpit and then another seam on the same side across the shoulder. I was able to pop myself out of it. We also found that it helped to lay on the floor on my side. Gravity pulled my skin away from the side so that my dh wouldn't cut me. We used the type of scissors that paramedics use to cut fabric etc. out in the field. I think that any heavy duty scissors would work. I then let the casting dry. Before it dried I reshaped the seam areas back to the shape that they were before cutting. You can either put it back together dry or wet but I found it worked ok dry. I used more plaster strips to put it back togeter. At the same time that you are putting the seams together you can cap off the arms and neck. I left the bottom open for the time being.
To prepare the stand I simply made a stand simular to a clothes tree... I then inserted the 2x2 up the middle to the neck. To help support the strain put on the neck area I used a plastic container that fit in the neck and then screwed the wood post through the plastic container and plaster neck. I used a wooden drapery end to cover the screw. I turned the whole thing upside down and supported it in a box so that is sat straight. I then injected foam insulation in it. You can find this in most hardware stores, it comes in a can and I think that it took two cans for me. I made sure that the post was where I wanted it until the foam dried. I then leveled the foam on the bottom and capped it with more plaster.
When the plaster is dry turn it back upright. I then sanded the plaster casting down to a smooth surface. Don't sand it too much. I then covered mine with black rib knit. I used elastic at the bottom so that it would fit over the bottom. I first covered the bottom with extra ribbing. I found that I had to fit the shoulders and neck from the wrong side and then after sewing I flipped it out to the right side. I used small pins around the areas where the knit did not fit right up against the form. I hammered them in through the plaster.