Add about 1/6 of a teaspoon* of CMC to 80 g of light yellow sugarpaste. Knead it well. Roll it out to about 5-6 mm. Place it on a piece of greaseproof paper so that the nice (smooth) side is touching the greaseproof paper (this will be the visible side so you don’t want cracks on it).
Apply a thin coat of piping gel on the flat side of the styrofoam**. Place it on the rolled out paste. Press gently.
*The paste you are using might require a different amount of CMC. Keep on adding it until the paste starts to become a little thicker. I always add the CMC shortly before rolling out/shaping the paste. If the paste is too soft (no CMC) it might be difficult to get clean/straight edges and a smooth surface. It will also take a long time for the paste to dry - it might get easily indented and you will have to wait long f.ex. with drawing the smile.
**If the surface of the styrofoam has holes, fill them with pieces of sugarpaste. If you leave them they might show through the sugarpaste (the surface won’t be smooth).