Easiest way is to use Cavalier thin resin zimmerit. Many vehicles have specific Cavalier sets for use on. Verlinden also does the same thing but with photo etched zimmerit.
There's also a water soluable putty called zimm-it-rite or something like that. You can wet it with your finger while forming it. There are also several tools to get the right pattern. Verlinden, Tamiya, Gum Ka, and several others make zimmerit tools for a variety of vehicle type patterns.
HTH