Step 1: Materials
You will need
1) A projectile that will fit the pipe (I used a spring, mostly because it fit nicely and had some good weight to it)
2) PVC pipe (a couple of feet should be good, the tile in the picture is 13 inches for comparison)
3) PVC cap (make sure it fits the pipe)
Step 2: Put It Together
Step 3: Load It
Step 4: Throwing
OK, this is the slightly tricky part. Remember we want to spin the pipe so what ever is inside slides out the end. Swinging it is very much like swinging a pick, but stopping early. It's easier if the projectile is started moving down the tube a little bit before the swing.
The motion is relatively simple, start with one hand about halfway up the pipe and the other at the bottom end with the cap. Swing the top end forward pushing the pipe way with the upper hand while sliding it down the pipe towards the bottom and stop right as it it infront of you. If timed right and swung with enough force it should come out the end.
When I get around to it I'll make some animated gifs to better show how it's done.