It's the same process as the Cradle Velocity Reducer
YouTube - Cradle Airsoft O-Ring Piston and Velocity Adjuster Installation for KJW M4
But IMO, the NPAS sucks. Mine broke in a matter of days. I would personally stick with the cradle one.
World record in propane sniffing.
-Tokyo Marui M4 MWS GBBR
-ICS CXP M4
-KJW KP07 (1911 MEU)