Take one used Christmas card. (Thank you Isolde for the card, and a Merry Christmas to you too!)
Cut it on the crease:
Fold the picture side (this will be the top of your box) in half, but only pinch it on the end where the arrow indicates, do not crease it all the way down.
Open the card and put a small mark where you pinched it.
Fold it lengthwise and do the same thing:
You should have all four sides marked:
Fold up to exactly your pen marks and crease, I used a sharpie to really push it down. Fold, crease, and open all four sides.
Cut from the edges where arrows indicate:
Fold sides in:
Tape or glue flaps:
You want the bottom of the box slightly smaller so the top will fit over it. You'll do the bottom almost exactly the same as the top except you want to make your folds slightly over your pen marks:
Tada! A box:
If you want your box to look not so creased and a bit more professional then I suggest measuring instead of pinching the folds. You could make a stack of these boxes and give them to the kids or you can fill them up with small Christmas gifts for next year :) Have fun!