Left - Using an oil based or polymer clay, model your figure over a wire armature. This figure is 22 inches.

Right -  To make the mold, place a thin strip of clay around the outside perimeter of the figure to divide the piece front to back. The front side of the mold will be made first, so place sheets of clay across the gaps (e.g., between the legs, in the underarm opening) from the back of the figure, securing the strips of clay with smaller pieces of clay. Press firmly, but not so hard as to distort the surface of your clay figure. The pieces will pull off cleanly when the front mold is finished. Push keys around the flange from the front side about every inch or so. The keys will help to lock the two parts of the mold together. 

Left - Spray the piece liberally with a mold release and then brush on four layers of rubber after the previous layer has set. This is Polytek's Polygel 40 and 50 urethane rubber. When the rubber has cured, quickly mix a batch of plaster the thickness of frosting and build up any undercut areas. Spray the plaster undercuts with mold release.

Right - Cover the entire side with the thick plaster mix. While still wet, lay pieces of wet burlap onto the surface for added support. When this layer has set, mix another batch and build up the plaster to an even thickness of about 1 inch over the entire front all the way out to the edges of the flange.

Left - When the plaster has set up, you can remove the strips of clay from the back that were used to divide the mold.

Right -  Since the figure is secured into place with the rigid plaster mold, you can now remove the armature from the back of the figure and do touch up modeling on the hole and surrounding area where it was attached. Once remodeling is complete, spray the back of the figure liberally with mold release (including the now exposed rubber edges of the front mold). The back of the mold is then built - rubber layers, mold release, then plaster layers. 

Here, both sides of the mold are complete. Let the plaster harden overnight and the next day, carefully pry into and loosen the ceramic shell with a screwdriver or crowbar until you can wiggle one side or the other off. Remove the other side of the plaster mold. Using a pair of sharp scissors,  cut around the perimeter of the rubber mold dividing line about 1/3 to 1/2  inch. That will leave a clean edge line to separate the two halves of the rubber mold.

To make a casting with your mold, open the rubber mold and spray both sides well with mold release. Silicone rubber molds usually will not stick, but here I have made a urethane rubber mold which will stick like glue to a polyurethane resin filling if no mold release, or the wrong type of mold release is used. Be sure to test the brand of mold release you use to ensure that the fill and rubber do not stick. 

Lay the pieces of the rubber mold together tightly in one half of the plaster mold and then lay the other plaster side on top. Wrap the entire mold with straps of inner tube and some sort of strong twine. Invert the mold set in a bucket and mix your fill. Here I am using a polyurethane resin (Easyflow 60) - one part A, one part B, and two parts porcelain powder. Mixing several batches, one after the next, fill the mold to the top. When the mold is half full, pick it up and tip it from side to side slightly to get rid of any air pockets that might be trapped inside. The figure can be unmolded in about an hour.

I decided I didn't like the way the towel she had draped over her arm was hanging, so I cut a piece of real towel, soaked it in a cup of the porcelain mixture, arranged it in place and let it harden as part of the statue. 

For the finish, I will cover the piece with white wax with just a pinch of iridescent powder mixed in. Below - finished casting from second mold.

You can rework any of the castings you make - a little, or a lot - and make another mold of the revision.

Even if you ruin your mold on the first attempt, all is not lost as long as you have made at least one half way decent casting from it. I decided to make some changes to the piece and make another mold, this time using silicone. Starting with the last bad porcelain casting from the previous mold, I removed the parts with a hammer and chisel that I wanted to change. This included the hands and towel, which were removed all together. 

Next,  I covered most of the piece with a very thin layer of plasteline because I wanted to change the surface treatment and the face, feet. and base. You can even melt some of the plasteline in a microwave and paint it on the cast porcelain piece with a small brush. Since the piece is rigid, no support system is needed and you can easily rework all of the parts of the piece that you'd like. 

Here we go again. Same process, just using a different material for this mold. This is Smooth-On's silicone rubber, Mold Max 30. Shown is the most important layer of the mold as the first thin "bubble"  layer captures the tiniest detail. This layer should be applied with a small brush to ensure that no air bubbles are trapped between the model and the rubber. Subsequent layers can be applied with a larger brush, spatula, or even a paint stirring stick

Since there is no support system now and the piece is already rigid, when the front rubber has cured, the piece can be laid down flat (on some pillow batting)  to make the back side without worrying about distorting the figure. Then the two sides of the plaster mold can be made. This is much easier than having to fight gravity with the piece standing vertical.