Vallejo has several washes already premixed, but if your dealer doesn't stock them you could always use what you have. In a mixing cup, put in a drop of black and two drops of thinner, and paint it on a painted test piece to see how it flows and dries. If it looks too dark, add in more thinner, too light more black. If you want a grimier look, mix in some rust colours. Experiment.
TazMan2000