Since this is very easy to make I will attempt to describe this in as few steps as possible.
Step 1: Cutting the Form
Cut half of the top sections. If the carton has a plastic screw top, simply rip it off.
Cut each side, next to the main panels, in half.
The product should look like the second image
Step 2: Folding the Seams
Next get cards that belong in your wallet to size up the card sections. Fold along this seam also. Follow the picture for the next three cuts. The first cut will be a "V" in the top flap - this will aid in the wallet actually folding together. The other two will be on the side so that the wallet can hold together without glue.
Step 3: Folding Together
You may find it useful to crease and score the edges to make the wallet sit flat. I use the handle of scissors to flatten them out - and it seems to work well.
Now fill your wallet with cash and go and spend some money!