Basically, its pretty simple.
First you want to get set up ahead of time and plan out how youre going to hold the gun and its smaller parts (if you take it apart to do it) while you paint, and hang them when youre done. A little prep here goes a long way. You dont want to start painting and then realize you dont want to be holding it until it drys.
Before you start, degrease everything well. I use Gun Scrubber, but something similar will work fine. I also degrease my hands just before I start.
I normally spray the base coat in the lightest color in the pattern, and then progress from there, lightest to darkest. I usually use a hair dryer between coats to help "set" the paint. When Im done, I give the whole gun a couple of three coats of Testors clear flat lacquer to help flatten out the whole paint job and give it some extra protection.
Some patterns can be sprayed over the base with a template, others do better with a brush.
For paint, I mostly use Testors Military Flats, as they offer about any color you might need to match a pattern, and they are removable (with some work). Ive also been using some Krylon lately, with good results to.
Brownells has some good videos with some pointers. Their Aluma Hyde II paints are good paints, but are more or less permanent. They work well for a base, and you can remove other paints while the base stays in place.
http://www.brownells.com/.aspx/lid=11837/GunTechdetail/Brownells_Spray_On_Paint_Series
My last couple have been in Multicam, or as near as I can get to it. Everything was done with spray cans, using a sheet of printer paper with patterns cut out for the patterns.