how to do a fondant box cake?
I can't think of a way to explain this to you in a short space, not because it's so hard, but because I'm not very good a saying things in a nutshell, but there are some great tutorials on how to do this in different places on the internet. I'd suggest that you google something like "gift box cakes" and see what turns up. Best of luck!
Sugarshack has a great DVD called Boxes and Bows. Basically, you cover the cake in 4 panels that had time to dry to stiffen so you get sharp edges.