Well, mostly I use USN BuShips camouflage pattern design plates, such as
this. Overlaid as a layer in Photoshop it provides a handy guide for where to put the camouflage colors. After that it's just the tedious task of filling pixels with color.
For some of the earlier patterns though it's basically a problem of looking at pictures and approximating it as best you can. This is usually the case with Measure 12 modified as there was no specific way to do it.