I have done several for friends after they see mine.
Pick a base color. I was after a waterfowl pattern last time so I used tan, coat the whole thing with it. Pick your next color, then place the nozzle very close to the gun and pass it over it quickly in random directions. Make thin ralatively sharply defined lines that crisscross with no pattern. Do this with 3 or 4 drab colors like flat tan, olive drab, flat brown, and flat black.
For a spring turkey I would suggest black over everything, then tan sparingly, then brown and then green. Going with the darkest color as the base, highlighting it with tan and then putting some woods colors on creates the illusion of depth and resembles the colors seen in the spring woods. Remember, all you are hoping to do is take the shine off and break up the outline of the gun. Taking away straight lines is the key.
The only way you are going to get a commercial camo pattern is to have a commercial outfit dip your gun at a cost of about $200. With about $15 in paint you can get an effective camo finish that a turkey/deer/bear/duck etc can't tell from Real Oak Superdifilous Woodsy pattern.
Grab a cardboard box, say a foot cubed. Test your idea on it and see how it turns out, you have six tries per box to find what you want.
Last thing I can think of is put a strip of masking tape down the rib before you start. I like a flat black rib and white bead/s. If a critter can see the solid line of a black rib it is too late for it so don't let that concern you.
EDIT: Whoops, forgot to talk about paint. Krylon has a camo series that works nicely. About $3 a can at Wal-Mart and other discount retailers. HS Specialties makes a paint that isn't nearly as good for about triple the cost, sold at most outdoors shops. Out of several brands and types I like the Krylon the best.