If you wanted to do urban camo, I'd figure codex grey fatigues, with splotches of shadow grey, fortress grey, and black. You said you wanted to do desert camo though, which is a little different. I'd probably do a bleached bone base, with graveyard earth, bubonic brown, and scorched brown splotches.
Camo is very easy to paint. It's really just random splotches in close proximity to each other. Here's the camo I painted for my desert themed Ork army, Da Dezz'urt Ratz: