DISCLAIMER: NEVER fire any gun without the proper ear and eye protection, and ALWAYS practice proper firearm safety. I am providing these instructions as a fun project, and I am not responsible for any injuries or broken laws that are a result of my instructions.
Step 1: Get a shotgun shell
Go to a range, and either fire a shotgun or collect the shells from someone else.
Fire your own shotgun, keep the shells.
Ask a friend for any shells they may have.
Step 3: Prepare to roll
Step 4: ROLL!
Step 5: Tighten the roll.
Take your left thumb and at least two fingers and put them on opposite sides of your roll.
Bring your hands together, relax one hand, and put them back to their original position. Repeat until you've reached your desired thickness.
Step 6: Put it in!
Step 7: Awesome!
WARNING: Please don't go through airport security with this, and don't try to buy a gun using this wallet. The owner of the store could mistake it for live ammo and it could turn ugly for you.