Step 1: Measure and Cut
The box will be sitting on its side when we're done, so pick your orientation. The top flap will be cut at 4 inches from the top (which means you'll be cutting off about 2 inches). Measure and mark a line 4 inches from the top on the top flap and 4 inches from the top on the side flaps. The side flaps are vertical, so we're not cutting these pieces off we're making horizontal cuts to make two flap pieces on the same side. (May need to skip ahead to see what we're doing as I may not be explaining this super clear) I found that if I cut the side flaps about 3 7/8 (almost 4) inches from the top it all lines up a bit better, but I plan to cover mine in white tissue paper so you won't see if they're lined up exactly.
I then found it necessary to cut a small strip off the bottom half of the side flaps and the bottom flap (about 1/4") so that they would fold inward without hitting the bottom of the box.