Step 1: Too Easy... Time a Minute
2. Feed a small (cheap ass tiny) water balloon through the tube but leave the top sticking out enough so you can pull it over the tube and secure it with a small rubber band. (see the diagram)
3. You'll prolly need a some sort of push rod to get a BB through the barrell and into the "sac"
4. once in the sac, just pull back and let go!
I never tested the accuracy all I know it hurts and its REALLY really tiny... only reason why I posted it. :)