The following photos illustrate how I approach sculpting a relief with emphasis placed on how I start such a project.
Figure 1 & 2 I first take a piece of floor tile that will accommodate the size of sculpture I plan to make. The tile is given a very light coat of Vaseline so the sculpture will easily separate from it once it's completed. Two pieces of wood or plastic strip stock is then placed on either side of the tile. It's important that the height of the stock allows it to sit above the surface of the tile. This will be used to determine the thickness of the relief.
Figures 3 & 4 A large piece of Sculpey was rolled out and then press down onto the tile. A piece of Baking Parchment or Wax Paper is then placed over this. A second piece of tile is pressed down onto the Sculpey positioned on the tile underneath. The strip stock on either side will ensure that the Sculpey is flattened to a uniform thickness. The Parchment or Wax Paper prevents the Sculpey from sticking to it.
Figures 5 & 6 I then peel the Parchment or Wax Paper off and replace it with a black and white print out of the subject. Keep the strip stock in place as the black and white image is then pressed down onto the Sculpey so it adheres to it to allow transfer of the image to the Sculpey.