To use a silicon cake decorating mold, dust the inside of the mold with cornstarch, tap the mold lightly on a hard surface to remove excess cornstarch, and press fondant into the design on the mold. Use a sharp knife to remove excess fondant, and smooth the fondant before gently folding the mold to remove it. If the mold design is intricate, place the mold in the freezer for five to 10 minutes before removing the fondant.
After removing the mold from the freezer, fold it gently to release the fondant, and allow the fondant to dry for 10 to 30 minutes before placing it on the cake. Use the tip of a sharp knife to release one edge of the fondant if it is difficult to remove.
To use a silicon onlay to decorate a cake, prepare the fondant, and use a rolling pin to flatten the fondant until it is 1/16 inch thick. Place a piece of the fondant on the overlay so that it extends past the edges of the mold, and press the fondant into the mold with a rolling pin.
Remove the unwanted pieces of fondant from the mold to reveal the design, and place the mold gently onto the side of the cake. Bend the mold away from the cake the release the fondant.