I have sooo many problems covering round cakes with fondant - but seem to do ok with square cakes - which seems to be totally opposite of what everyone else says! (Leave it to me to be bass-ackwards!) Anyway, what works best for me is to elevate everything. I have the cake on my turntable, I set the turntable on top of a Criscoe can (the can somewhat fits up inside the base of the turntable so it's pretty secure) and then the whole works goes on top of a pedastal cake plate. When I first apply the fondant, it hangs down past the turntable, and then I use my hands to smooth it as I turn the cake, slowly.
Then I use a pizza cutter to cut off the excess, a little bit at a time all the way around.
Like I said, I don't know why, but using this method has helped me cover square cakes pretty nicely. And, I just started making Marshmallow fondant about a month ago! I have been so excited with how well it works, looks, and tastes! (Will be loading some pictures in the next few days!)