Your best bet, is to paint the base coat in the lightest color, and then for the most part, go lighter to darker with the rest.
Before you start, you want to do a little prep. Make sure you degrease everything real well. I usually use Gun Scrubber or something similar. Do everything a couple of times, and do your hands as well, especially just before you paint.
Youll also want to figure out how you want to hold the gun while you paint, and how you will hang or prop it up to dry. Dont wait until you start painting to figure it out, do a dry run ahead of time.
When you paint, dont try to do it all in one coat. Do a couple of three or four light coats, and use a hair dryer on it after each coat to "set" the paint.
If you go to Brownells, they have some videos on how to apply their Aluma Hyde II, and some of their other paints. They are well worth watching, even if youre not using those paints.
Aluma Hyde II is a good paint to by the way, but it is pretty much permanent. Ive used it on a number of things with very good results, but you do have to follow the directions to the letter, and it takes about a week to cure, before you can touch it.
Mostly, I use Testors Military Flats, and lately, some of the Krylon "camo" colors. The Testors can be removed with some work, and the krylon seems to come of OK as well. Their "Fusion" line is made to bond with plastics, and may not though.
I used to break the gun down into parts and paint everything that way. I no longer do that, unless Im looking to use the paint as refinish. These days, I just tape off the sights and anything I dont want painted, and then just spray the whole gun in the base color. Then I take sheets of printer/notebook paper, and cut random patterns in shapes similar to the pattern I want to duplicate. Then I lay them on the gun where I want them, and make a couple of passes over them with the next color paint, until I get them all done.
This was the last paint job I did. Its my M&P15 in Multicam........
Same gun on the pattern....