Find a box that is roughly this shape. We got this box from a comic book shop. It's nearly perfect. 7x14x10.5". Box size really isn't that critical but after cutting ~10 boxes this one seemed to work best. Of course, you could make your own box.

Tape the box completely shut (we neglected to do this starting out; learn from our mistakes!)

Take your piece of glass and use it as a ruler to mark the diagonal line from corner to corner.

Cut across the diagonal.

Do the same thing on the other side.

If there are any remaining parts of the box hanging free, tape 'em up. If you taped your box completely shut at the beginning, you shouldn't have these kinds of problems.