In this video I show how I modified the sprinkler valve for my air cannon    • Making a compressed air cannon  
and my Nerf cannon    • Making a nerf cannon (work in progress)  

It's pretty simple. Just two main steps.
1. attach an air gun to release the air behind the diaphragm.
2. seal up all other air inlets

Amazon links
Air blow gun - http://amzn.to/2r31pJ1
1/4 male air pipe connector - http://amzn.to/2r1qumP
Sprinkler valve - http://amzn.to/2q4de3r (or in Australia - https://www.bunnings.com.au/holman-25...)