when you are smoothing out the folds that occur as your lay the fondant on the cake, make sure you smooth the top, then smooth (at this point just using your hand) from the top straight down (not side to side). You use one hand to lift the bottom slightly, while using the other hand to smooth it top to bottom. Then continue on along the cake.
It also helps if after you lay the fondant on you tim the bulk away (if you have any) before smoothing. Once you really get the hang of rolling out the correct size it won't be an issue, but for me, this was a crucial step at first. I cut away all about about the bottom inch. Then smooth. Then do the final trim.
And DEFINITELY make sure the cake is smooth before applying. Any ripples in the buttercreamor lumps ni the cake will be magnified by the fondant. I use fondant cakes as practice for getting smooth buttercream. I make it as smooth as though I were not using anything else, THEN I put on the fondant.