This is basically a staining paint technique and it works well for speed painting large numbers of small figures. I have used this on 15mm and on my 20mm WW2 figures. It is great for 20mm plastics as these are always chipping and need frequent repaints.
The first step is to clean the figures and remove any flash or mold lines. Be very careful. These guys are small and it can be hard to tell the difference between a detail and flash. Take your time. I like to mount the figures to a Popsicle stick for painting.
After the figures have been cleaned it is time to prime. This technique requires a black undercoat. Unless you are some kind of painting genius do not spray prime these. Spray paint will destroy the detail on these small figures very easily. It is best to do a brush coat of plain black paint.
Once the black base coat has dried thoroughly it is time for a 'dry' brush of white. This is not like dry brushing a tank. For this coat I use a little 'wetter' dry brush. What you want is to bring out the details by creating a high contrast on the model.
After the white has dried it is time to add the basic colors to the model. This bulk color should be applied with thinned down paint. I thin the paint until it is like milk. It is important to get the right consistency. Too thick and you will obscure the contrast layer, too thin and the paint will not color the area well. The idea is to get a transparent paint that will give the correct color but allow the contrast through.
After the base colors are on it is time for some details. For the details like flesh, belts, boots and guns, use paint that is not thinned or thinned much less than the bulk areas.
At this point the paint is done. No really, these guys are done. Yes, I know I have missed a bunch of spots and if you have missed stuff feel free to go back and clean things up as much as you would like. I am lazy so these are done.
Long time readers know that I am a 'dipper.' I use Minwax Polyshades wood stains on most of my projects. For these little guys I am using the Antique Walnut which is a red-brown color. This will give the figures a nice earthy look and help hide some of my sloppy painting. I brush on the stain like a wash, I do not actually dip the figures.