Fondant is quite the tricky cake decorating material. On the surface it looks smooth, slick, and solid, but the consistency of fondant is actually quite thin and the sugary paste is very liable to tearing while you're working with it.
If you've never laid down a large piece of fondant on a cake before, then this video is a must-watch for you. You'll be guided step by step through the process of tailoring a rolled fondant to a square shaped cake. In this video a dummy cake made out of styrofoam is used, but you get the idea.