I had the same question a month ago and like nicunurse, got Sharon Zambito's Flawless Fondant DVD. Worth every penny. The first cake I covered after watching it was round but it had straight sides and sharp edges.
In the meantime, make sure you have sharp edges BEFORE you cover your cake with fondant.
I just finished watching a video on YouTube about covering a square cake with fondant. I don't know the lady's name and I didn't have the sound on, but I couldn't get past her long hair hanging down over the cake. It was very unappetizing to see her flip her hair back or tuck it behind her ear, and then touch the cake.
Please make sure you keep your hair out of the way.