1) Remove any excess plastic from the figures. Sometimes there's extra plastic at the seams where the two halves of the mold met, or at the points where the plastic was injected.
2) Use an old toothbrush and a cup filled with dish detergent to wash the figures. Rinse well, and let dry completely. I usually let them dry overnight. Washing the figures well is probably the most important step to ensure that the paint will adhere to the figures. This is because during the manufacturing process, the mold is coated with a non-adhesive release agent so that the plastic does not stick to the mold, and some of the release agent gets transferred to the figures.
3) Apply a good coat of primer. I've read that it allows the paint to bond better, but for me the main reason to do it is that it lets me get away with doing only one coat of paint. Otherwise, depending on the color of the plastic and the color of the paint, you would need to apply a couple coats.
-use large brush to paint skin fast, without regard for painting beyond the actual skin surface
-use large brush to paint uniform pants and blouse fast, without regard for anything at the bottom of the pants, but making sure not to get any uniform color on the skin.
-use large brush to paint ground fast without regard for shoes, but avoiding any legs on the ground.
-use medium brush to cut uniform neck, cuffs, and the pant's leg openings.
-use medium brush to paint and cut shoes.
From here on it's a matter of painting weapons, equipment and the finer details, but in general, you try to use as big a brush as you can to paint as much surface quickly enough, and then you use the smaller brush to cut the transition lines. A good rule of thumb is to paint only as much detail as you can see when you hold the figure at arm's length, which is what you will normally be able to appreciate when you look at them in the course of playing or putting together a diorama. It's also the amount of detail that you would see if a real man were standing about 50 ft (15m) away. Then again, if you are planning to shoot pictures close up, like the ones I have been making for these posts, you might want to paint a bit more detail than I usually do.
7) After you are done painting you have a good looking figure, but you might not be able to do much with it besides looking at it or else the paint might chip off. If you want to use them for gaming or you don't want to have to worry about how to store them or transport them, then you want to apply some kind of protective finish. What I have found the best solution so far is to cover them in a coat or two of liquid rubber. I do two coats, just to be on the safe side. Since the rubber is elastic, it allows things like rifles to bend, without the paint cracking and coming off. There is a great product to do this called 'Plasti Dip'. It comes in spray and liquid formats and in multiple colors. Buy the clear color, in liquid form. The spray version does not work as it comes out too thick and creates big clumps on the figure.
8) The only problem with the Plasti Dip is that well, it feels rubbery, so to give it a smooth finish I follow the Plasti Dip with a coat of liquid lacquer, the kind used to finish wood. I use Minwax water-based polycrylic protective finish.
9) Lastly, depending on whether I want the figures to have a clean look or a rough/combat look, I might apply a coat of blackwash. To do this I use highly-watered-down black acrylic paint.
Well, this post turned out to be quite a bit longer than the bullet point list that I initially conceived, but I think it contains most of what you might need. Good luck with your projects!