Originally Posted by Scissor Queen

That's not at all helpful for somebody that is folding the back over to the front to finish the edge.

One of the things you have to do is clip off a triangle on the corner leaving at least a quarter of an inch but not enough that it'll show after you have the sides folded in. Then you fold that edge down first over the corner and then fold the two edges so they miter in the center of the corner.

edit to add: I searched and found some good instructions for you with some pictures.