Thus, I made myself some boxes out of Fedex/UPS envelopes. I have such a stash of these things that I need to use up it isn't funny any more. If you have any ideas, feel free to let me know!
On a scrap of paper, write down your dimensions and add them together. I sketched mine out and added it that way, I'm much more of a visual person. My dimensions ended up (after a couple of boxes to truly figure out what I wanted) being 2.5 inches high, seven inches length, and five inches width. So I needed a length of 13 inches (2.5 + 7 + 2.5) and a width of 10 inches (2.5 + 5 + 2.5). With me so far? I made my box like a + and folded up the sides, so I had to add the sides to my length and width in order for it to turn out right.
I'll stop trying to explain math now.
I cut open my envelope and laid it out flat. Then I took my ruler and sketched out the square 10x13 shape. After cutting this out, I went back and made my actual lines at the appropriate spots. Once I had my lines drawn, I went back in and drew an additional line .5 inches out from the lines.
You can see the lines in the picture above. By adding in those lines, you are adding the flaps that will be folded over and glued into place. At the small box where all the lines meet, I cut diagonally out for the flaps.
Once you are done cutting, score along the inner lines that you will fold on. Then fold up and glue. I folded one flap on the inside and one on the outside of each corner.
Once the glue is dry, throw in some stuff and you have a box!