My main problem with modeling clay is that I cannot make two identical pieces without a pattern or mold. So, I have solved the problem of making heels by using a cardboard mold.
This time I was making heels for 16" Miss Piggy's shoes. The photo below shows the cardboard insole with an empty mold for the heel.
The hardest part is making the pattern for the mold. You can see the basic form below, but it is very much trial and error. Once you've got the pattern right, draw the required number of them on cardboard, fold the front parts as shown, and cut out the pieces. You can make the folds after cutting, but I find this order easier.
Curve the back and sides of the heels using a suitable round object, for example, a marker pen.Then glue the front as shown. The result is the neatest, if the lower flap (the one that goes inside, against the modeling clay) covers the entire width of the heel front. If it doesn't, you'll get a visible seam in the modeling clay (see the last photo of this blog post).
When the glue is dry, pack the molds full of modeling clay. Press it tightly to avoid any gaps, but be careful, so you won't crush the mold. When the mold is full, use a craft knife to cut off any modeling clay that exceeds the top or bottom of the mold, and make sure the form of the heel is the way you want it.
Bake the heels in the oven according to the instructions given in the packaging of the modeling clay you're using. After getting them out of the oven, remove the cardboard immediately (as soon as you can handle the hot pieces). If you let the pieces cool down, the cardboard will be a lot harder to remove. If you're going to cover the heels with fabric, leather, etc., you can also leave the cardboard in place (especially, if you have used leftover pieces of clay of various colors and want to make sure that won't show through the material used for covering the heels).
Here are the finished heels for Miss Piggy.
The top of the heel may be convex even when you have cut it before baking. If that is the case, just use a craft knife to remove any extra material. This has the added benefit that glue will hold better, if the surface is cut or sanded after baking.