1) Paper Towel Method:
Frost the cake as you normally would and chill for 30 minutes.
Using Viva brand paper towels (I tried it with Viva because they don't have raised designs) wrap the paper towel around the side of the cake gently and run a fondant smoother or an offset spatula over the paper towel pushing up slightly on the buttercream. Continue all the way around the sides of the cake.
2) Hot Water Method:
After the cake has been completely frosted allow the cake to chill until the outside is completely solid.
Pull out an offset spatula and either boil a pot of water and dip the spatula or run the spatula underneath the hot water of a faucet. Dry quickly and smooth out cake with the hot spatula, the heat from the spatula will melt away any faults.
3) Fondant Cakes:
After the fondant has been applied take two fondant smoothers and simultaneously smooth the top and sides making sure to pinch the edges of the cake with the smoothers.