Making a custom box can be used for a few good reasons that I have found, as well as- I'm sure it could be used for various other and different reasons. Maybe it is a holiday season and you are wrapping a gift that needs a box for shape or structure; or would benefit from a layer of protecting cardboard; or need to fit an item of an odd shape/size into a box for storing/mailing.
I make boxes for the items that I sell on eBay. Making an item's own box minimizes the size, the weight, the need for more packaging material, and thus making the postage/shipping fees less.
All you need is second hand cardboard, a box cutter (razor blade), and packing tape (in a dispenser works best).
Step 1: Break Down
Break the box down starting with opening the tabs on top and bottom of the box. Just unfold the tabs, or cut through the tape with the box cutter and then unfold the tabs. Lay the box down on its side and cut one of the corners from top to bottom so that you can unfold the cardboard once more to end up with a long, straight piece of cardboard.