That second guide in Boonie's post, that pretty much sums up the technique for doing it with the paint.
In essence it's spraying lighter colour from above to simulate light falling on the model and as simple as that sounds it's really not much more complex than that in practice.
Once you mess about with it it'll make sense. The technique doesn't work perfectly every time, since some shapes don't catch the paint the way you want (vertical grooves that should be dark for example can end up with light paint in them) so you do need to be willing to do touchups and revisions using a brush, but when you're lucky there's very little necessary. I generally like to do final highlights and the darkest shadows entirely by hand for the best results.
You can start dark and spray in one or two stages to the midtone and then on into the highlights but it also works sometimes to start at the midtone and work in both directions. Try both, see which suits you best. The thing you want to be sure to avoid - and you will get this unintentionally a few times - is spraying highlight colours onto a shadowed area or shadow colours onto a highlight area, since the resulting colour looks wrong (just as they do when painting with a brush).
For subjects unlike SMs that aren't basically all one colour you then need to look at masking, but that's another thread.
Einion
