Search the forum here, as there are some detailed exchanges with links on How-To videos on YouTube and elsewhere.
The keys are to wash the stock thoroughly to degrease its exterior, and to use a paint made for plastics, such as Krylon spray paints made for lawn furniture and for the military, in the camo colors.
Then you need to think backwards if you use patterns, whether purchased masking or sticks and leaves as masks. What I mean here is, that if you want a leave shape in green, you spray a spot there, then when it is dry, you cover it with the leaf and spray with the surrounding color, like brown.