ok, so i know this is crazy but i've only dealt with round cakes, how do i fondant a square cake and get the edges to come out smooth and without folding?
You basically do it the same way. Put the fondant over the cake and smooth the top. Then work your way down the sides a little at a time, just pulling and smoothing. It is better if you get the corners to stick first though because that is where it tends to bunch up.
Use two fondant smoothers to help you smooth the corners - one for each side.
What they ^^^ said and.........start with the corners and work the extra fondant into the centers.
Just make sure you stretch the fondant gently to cover the corners and they work inwards. Always use two smoothers as well, working towards each other to get the nice defined edges and corners.