I'm going to be detailed and basic, please don't get offended. I had the advantage of someone to show me this the first time and reading instructions isn't the same...so more is better than less.
The very first step is to get a cheap acetone, nail polish remover works fine, and clean the areas that will be painted, to get all the oils off.
Next you need to make the "stencils" for the camo pattern. What you want to do is rip painters tape (get the wide kind) into random strips. Think of it like ripping it to look jagged...you'll need two strips for one break-up...one for each side...the larger the space in between the strips the lager the break-ups will be. Tape the strips to the stock and around it to cover the non-paint areas. You can add smaller scraps inside the break-ups to alter the shape. where the tape is will stay black, the negative space will be whatever color you pick. Make sure the tape is on good so you don't get any spray where you don't want it. Any other area with no break-ups should be covered in tape so it doesn't get painted. That's the prep.
Next spray it, I like Krylon ultra-flat camo paint and Krylon matte clear coat.
Spray the color on first, take your time and spray even coats...wait a couple mins between coats (it drys fast) but not too long. I'd do 3 good even coats before I took the tape off. After taking it off, use the clear coat to seal the paint, I'd do 2 coats simi-close to the stock to get a good coat and 2 coats at a distance to take any shine away, the powder spay at a distance will do that.
And that's really about it...use stencils, you can make or buy them if you want...I just did a stock that I'll add a pic of where I added skulls, just for fun...I was thinking next time trying to put the skulls in the break-up. I think that would look good. Best thing about it is, no one will ever have a gun quite like yours.
I hope this helps...it makes sense to me, but I know how to do it. It's not always easy to tell someone when you can't show them...I think if you read this and look at the picture of mine, it will be clearer...Good Luck! and remember, it's only paint you can paint over it, or remove it.
Like I said I'll add pics of other guns I've painted to give you some ideas, I'd love to see it when you're done.