Faux cakes are often used for practicing decorating techniques or to have a larger cake on display than what is needed to feed guests. Polystyrene cake blanks are easy to decorate, and they can be reused for practice.
Attach the cake blanks to the cardboards with the wood screws. This adds weight to the lightweight blanks to make them easier to work with.
Sand the edges of the blanks to round and smooth them to avoid tearing the fondant. If the sides are not smooth or have dents or dings, sand them lightly, too.
Cover the sanded blanks heavily with vegetable shortening to fill in crevices and make a smooth surface for the fondant. Smooth the shortening and wipe away the excess.
Cover the faux cake with fondant and trim it to size. The fondant will shrink, so leave a little extra to account for that.
Cut out the fondant in the center of the top of lower tiers, leaving about 2 inches around the edge. This allows you to save that fondant for another use. If you are using a real cake layer, leave the fondant intact for that layer.
Brush the fondant with water or pipe some gel and stack the next tier on top.
Use buttercream frosting to make borders and embellishments or add silk flowers or other decorations to your cake.