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.