Just white spirit and the paint. The ratio is up to you, buit it might be best starting with a light wash, 10parts thinner, 1 part paint, and re-apply another one ot two, to obtain what you want. I use sepia, rather than black to do my 'overall wash'.
If you have enamel as base coat, make sure the paint is really very, very dry, or the wash will react with it...