If you are doing it at the same scale as the clothing (ie, the pixels being 1 - 2 mm in size), you would probably do a stencil, and then either use a stencil brush, or a spray paint. There are some really good examples of people doing this to rifles, mostly Americans in MARPAT or ACU colours
These are threads from Lightfighter (sort of the American equivalent to this board, but make an introduction in the correct thread in the Crusader Hall before you make any posts, you will probably need to register to see the pics.)
http://lightfighter.net/eve/forums/a/tpc/f/9046084761/m/373103967?r=405109828#405109828
http://lightfighter.net/eve/forums/a/tpc/f/9046084761/m/473101226?r=473101226#473101226
(This one has CADPAT versions) http://lightfighter.net/eve/forums/a/tpc/f/829100313/m/4011054171?r=6421008271#6421008271
This page has some great pictures of the stencils that he used:
http://www.unblock.be/index.php?q=aHR0cDovL3d3dy5taWxpdGFyeW1vcm9ucy5jb20vd2VhcG9ucy9hci5taXNjLmh0bWw%3D (I had to use a redirection site, as my work blocked www.militarymorons.com, but if you want the correct page, go to the site and do a search for "ar15 misc stuff")
If you were going to do a larger area (like a wall in a room), I would guess that you would make up a 'digital' grid on the wall, with light pencil marks, and then figure out which colour was going to go in which square.
Another idea would be to print a CADPAT picture on a mylar sheet, and then use an overhead projector to project it onto the wall, and use that pattern to figure out which colours go where. Of course, this one would be somewhat distorted at the edges, due to the 'stretching' of the picture from the projector.
If I was doing a larger area, like a room, I would be inclined to do it at a 5cm 'pixel' size, or maybe even larger. It's not like you are actually using it to really camoflauge something, so if it is not 'effective', it would not really matter.
I saw an SUV in Mission, British Columbia that was done up in a Jungle Stalker colour digital pattern. I had thought of doing it to my vehicle ;D, but my interior is burgundy, so it would not really match with the CADPAT colours too well. :-X Yeah, I'm a geek, what's your point?
So, yeah, probably do a base coat in the lighter green, then do highlights in the darker green, black, and brown. Of course, you will be painting 4 different colours, so like Freeze said, lots of time, and lots of liquid refreshment. And probably an appointment with a counsellor afterwards. Of course, I have 2 kids, so I am already certifiably insane.
Yeah, I have put some thought and research to this, I guess. :