Step 1: Materials Needed:
1: A spring. This needs to be stiff enough to fire the pelets a good distance, but not so stiff as to make it impossible to load. Best test is if you can compress it to half it's length with your bare hands, it'll work ok for this.
2: A length of PVC pipe. it needs to have an inside diameter that will fit your spring tightly, and a length to completely hold the spring when its not compressed, plus about an inch.
3: A 1/4 inch bolt that is 1.5 inches long.
4: Two nuts that fit the bolt's threads
5: A retaining pin. This can be any kind of pin, but I recommend getting one with some kind of safety devise to keep it from working it's way out. You'll see what I'm talking about later on
6: Some cardboard
7: Lots of ammo.
8: Some kind of tube or rod to fit inside the PVC but not inside the spring.
It can be made with a hacksaw and a 1/4 inch push drill, but a Dremel and a power drill help a lot. Also, unless you like the 'plumber' look, some spray paint helps.