An approach that has worked well for me as I use my "cheap-o" craft paints is to take a small container - plastic paint-mixing pallette, drink-mix scoop, etc - and put some water in it. Before the paint, I'll thoroughly wet my brush. Once I have a loaded brush, I'll dip it into the water again, almost as if I'm going to clean the brush - but very quickly. Then I'll apply the paint as normal, watching to see how the paint coverage ends up. Based on the results, I may vary the amount of paint on the brush, the amount of time I let it sit in the water, etc. I'll dip the brush into the water several times on one loading of paint, keeping plenty of water in the brush to thin the paint as I apply it.
Doing this, I use the water for ONE COLOR ONLY. No sense risking ruination of a good paint-job to save an ounce of water!
You can, of course, thin down your paint more traditionally. Take the aforementioned plastic mixing pallette and put a small amount of paint in one of the cups. Use a very small amount. Really. SMALL! Like start with a toothpick and put three or four (maybe!) drops of paint on it. Then, with a fresh toothpick, put ONE drop of CLEAN water alongside the paint. With a third clean toothpick, mix the paint and water together. Try applying a test-coat of the thinned paint. If it is too thin, it will run off your surface and puddle in the crevices. This is called a "wash" and we love these for the LAST step of painting - but not for the first. If this is what happens, add another drop of paint with the first toothpick and mix again with the third toothpick. Odds are, though, your mix will be just right to a touch too thick still.
If your test-patch dries and shows brush-strokes, add another drop of water with the second toothpick and stir with the third. Your goal is a paint mix that covers well without running into the crevices (too thin) or holding brush-strokes (too thick). Start small to avoid waste and to make managing the balance of paint to water much easier. If you added more water and it tries to run off your intended surface, add a tiny bit more paint. Toothpicks will become your best friends and soldiers, like paws on a chessboard. You will use a LOT of them. Just keep them segregated by task - gross paint-transfer, gross water transfer, and mixing - and they will serve you well.
Once you, your brushes, and your paints understand how to work together like this, it's time to start looking at things like basecoats, washes, drybrushing, and the like.
Practice a lot. Change things up in small amounts and see what you get. Experiment. When you find something that works for you and gives consistently attractive results, make it YOURS!
And always always always ask questions - of yourself and of the folks whose works you like. And, conversely, when someone asks how you pulled off an effect, be ready to share.
Oh, and practice.
(On a general note - do NOT get discouraged if you set up a GREAT-looking figure and take pics ... only to have the camera reveal 16 flaws in him!! That's normal. I am continually amazed at how my camera and I can look at the same subject and see such radically different things! Use that feedback in the above experimentation stages. And practice.)
