Just do a crumb coat in green or brown, that way no little areas will show through. Then have brown, black, light green, and army green in bags ready to go. Just pick a starting point and pipe an odd shape using one of the colors and a round tip like #6. Use another color and pipe another odd shape going off of the one you just made. Try not to let two of the same colors touch. After you have your area covered and it has formed a slight crust, just use the Melvira method (foam paint roller) to smooth it out.
It's really easy to do in fondant. Pick one of the camo colors as your "base" and roll it out partially, brush off as much of the PS/starch as possible, then add small blobs of fondant in the other camo colors you want and roll it out the rest of the way. There you go!
One of my baker friends does a lot of camo cakes with her airbrush. If you don't own one of those, what about the cans of spray food coloring. Just smooth on a coating of buttercream then spray alternating colors in a random manner. HTH
I've done a few of these and they are so fun because there is no way to mess it up. One customer wanted theirs done with the star tip all over. I mixed tan, forest green, juniper green and black and put each in a bag with the star tip. Like the others said, just pick a starting point and make different sizes and different shapes all over, even down the sides. I've also done them with the larger round tip and once it crusted i just got a sheet of copy paper and smoothed it out.
I only put a bottom border on mine in the tan since they just wanted camo with no other decorations. I did print "happy birthday" and their name on the top of the cake with a larger round tip.
Just use your imagination.
Good Luck! 
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