Well most of us use acrylic paints. Most guys use citadel, or tamiya, and theres a couple other brands. I use folk art and delta acrylic paints myself. Mr Uruk recommended washing the figure, which I didnt use to do, but I did on my Ultimate Elder figure and it helped the paint apply and stick much better, so I think washing the figure is a good starting point. Usually I put them under hot water and take all the pieces apart, all the armor, dreads, etc. its much easier to paint this way. Just pick your colors and patterns, and good reference pics, and go at it. Dont be afraid, if worse comes to worse, paint over it, or put it under hot water and scrub it off with a toothbrush to start over. Let the paint dry in between coats, dont rush it. Usually when doing a custom, picking some kind of theme inspires your design... for instance, ive seen gladiator predators, battle predators, variation of movie predators, etc...
You can also interchange some parts...arms, torsos, legs, feet... my elder has his own head, arms, and legs and feet, with a p2 torso for the added height... be creative.
~CYCLONE_06