Check out camomyride.com and you'll get some ideas & how-to there. There's another site, don't remember the name but found it searching for "camo patterns".
A buddy & I did his Blazer this way: shot it with a dark flat green, then using edges of construction paper laid down some tree trunks & branches, in black & dark brown; made a bunch of leaf stencils (different leaves), and used different shades of browns & reds. Last, we highlighted some of the trunks & branches with a light tan & touch-up brush. We spent maybe 20 hours on it. Looks great. Only the ground coat was sprayed with a gun in our booth; the rest was rattle cans. Gonna do my plow truck this summer in similar fashion. I'll probably shoot a satin or flat clear over mine for UV protection, though his is holding up well.