As I’ve said elsewhere my son has been working on painting his own Space Orks. The method that I learned for painting minis, especially many minis, came from one of the studio artists who worked for Games Workshop. When you are painting a unit of troops that are all going to have similar colors in their uniforms and equipment paint all of one particular color onto each model. Also, start with what’s farthest away from you, such as the skin of the model. In the case of the Orks that means painting all of the units skins green. Then work on the next layer of clothing or armor up from their skin and do all of those, and so on outwards. I usually save eyes and teeth for last.
Here are step progress shots of some of my son’s troops;