I just summed up everything in a single picture, as going step by step would make a much bigger file. Anyways, since every single important tools I use for this kind of work appear here, it shouldn't hard to set yourself up. The real challenge is to make random shapes like the ones in the red sample pattern.
Anyways, good luck to any of you who wants to give a try on those pesky digital camouflage patterns!
Photoshop CS5 & Wacom Intuos4 Med
*Since other graphics editing softwares (GIMP, etc) can use brush packs, trying looking around for the square ones, unless they come by default already.