OK, first of all, I use a black base coat. Some people say that washing the figures in a mild soapy water before painting helps to remove the mold release agents and finger oils that may be on the figure. That may be so, but so far I haven't taken the time to do this. I'll probably regret this later...
Anyway, a black base coat applied full strength out of the bottle is the first step. Wait for this to dry thoroughly (over night at least) and then do a HEAVY dry brushing with flat white acrylic. Enamels may well work for this technique but I've never tried it. The dry brushing should leave most of the flat surfaces white, leaving black only in the recesses and creases.
The colors are added by using thinned acrylics. I thin the color I want to the consistency of milk (roughly...it depends a little on the particular color and brand of paint...try it and you'll get the hang of it very quickly), then apply it right over the white and black for the relevant area of the figure. The paint should stain the white areas to the appropriate color, and tint the black areas. The result is a nice shading effect. I use straight Steel colored paint for gun metal or other metallic areas. A stain of metallic paint really doesn't work well at all.
The finished figures are, in my case, mounted on fender washers (steel washers about an inch (i.e. 2cm or so) across with a very small hole in the center), and the bases are flocked. The whole figure is then sprayed with a flat varnish.
I paint large groups of figures at the same time using this technique, applying one color to all of the figures before moving on to the next color. It takes me about four hours to paint about forty figures with simple color schemes (American WWII infantry, Russian WWII infantry, etc.) or twenty or so figures with more complex color schemes (British AWI Highlanders, any figures with camoflage patterns).
Oh, one more point: areas of flesh look better if they are treated with a brown wash after the flesh tone has dried. It tends to "warm up" the face and hands and looks much more convincing.
When I first tried this technique I despaired about half way through the first batch of figures. The colors all looked washed out and all the remaining white made me think the figures would never turn out. But, once all of the white areas (that weren't _supposed_ to be white) were covered the unified whole really looked nice and I've stayed with the technique ever since.
This technique also apparently works on 15mm lead if the surface detail is well enough defined. I recently described this technique to a friend in Kansas City while on a business trip. The next time I came back through he showed me a collection of the new 15mm Old Glory WWII Germans that another friend had painted using the technique. They looked very nice! I can't tell you if he did anything different on those figures though, so there may need to be some adjustments to the idea for use on lead.