This is on a Savage 7mm Mag but its the same process for a shotgun.
Step one
Buy Kyrlon ultra flat camouflage spray paint. One base color and one dark color for the accent colors. On my semi auto in the first picture I used desert tan and olive while on the shotgun with the duck in the stock of my other thread I used Khaki, khaki is a little darker than desert, both of them I used an olive top coat though. I like them both, and honestly I dont know what one I prefer more. Its all up to you.
http://www.amazon.com/home-improvement/dp/B00176UX26
Heres the paint. You can easily buy it at walmart or home depot or any place that sells spray paint.
Also you will need a corn broom. I used little corn brooms made for dusting off horses, but you can use a full size broom, I just happened across some small ones real cheap.
Step two
Take the corn broom and cut chunks off of it about a round as you can fit in your fist. Then tape the bottoms to hold them together, use duck tape or something strong and be sure to tape it tight.
