Like SandyT, I mist my cakes with water from a spray bottle before I cover them to make sure the fondant sticks to the crusted buttercream. Then after making sure the fondant is snug around the base and sides I trim with a pizza cutter. If its a shape the pizza cutter cant handle then I use an exacto knife :)
Nikki, So Cal, www.Facebook.com/nikkibelleperchecakes
