I have a hard time throwing cards of any kind away...so a few years ago I started making these boxes with all the cards of Christmases past. They work great for all the little ornaments and such that the kids make to give for gifts. It's good measurement practice for my older two, not to mention an exercise in precision.
Step 1: Get your card. And separate the front from the back. The front of the card will be your top half of the box and the back will be the bottom. Step 2: Measure. You want to mark your measurement on each side- I usually do about an inch, unless the card is small or I need a larger and narrower box. I make the bottom just a hair smaller than the top so that the top fits nicely over it.
Step 3: Once all your measurements are marked, connect those marks on opposite sides to make the grid for your box.
Step 4: Next you want to snip on one line of each corner. I usually go every other line all the way around. This creates a flap that will be used to assemble the actual box.
Step 5: Fold. I use the edge of the ruler to get a good crease....if you happen to have the tool made especially for that purpose it would work too. Fold up each side, making sure to crease each of the of the flaps too.
Step 6: Assembly. Fold up each side one at a time. Fold the flap on the inside and tape to the adjacent side to create a corner. Once the insides are taped place an additional piece of (invisable) scotch tape on the outside of each corner.