How thin the wash should be is a matter of experimentation. I use Isopropyl rubbing alcohol in a eye dropper and thin it as I go. Demineralized water works as well, tap water will leave stains as it is not pure. The Isopropyl evaporates fast so have a bit on hand to 'top up' The wash should darken the recesses & shadows without changing the entire color. Try just painting it on where it should be, doing the entire figure in one swipe kind of defeats the purpose of 'shadows & highlights'.
I will post a few figure pictures for you to examine, my work is far from 'master level yet they turn out convincing:
First thing first, red oxide primer and basic skin tones. I do not like the 'flesh' color, add brown and or red to it.
The rest of the basic colors, not heads remain off as they can be added later with ease of shirt collar painting now.
Shown are my 3 colors for skin tones. Washes for recesses & shadows, highlights dry brushed.
Pencil crayons are handy to add details like eye brows, mustaches, the 'shine' in the leather jacket, Lead pencil for metal zippers & badges.They work better over flat paint as its rougher than gloss and will 'grip' the pencil crayon pigment better.
First for me, eyes added with a white 'wax industrial crayon. It is for marking fabric, plastic etc. The genius of it is it 'chips off with a sharp tooth pick, 'dab' the white on and trim to size. Blue added with a blue pencil crayon. Hair shine is future brushed on.
Figures on a suitable base, later will be added to a P-51 under repair.
I hope this quick tutorial helps you, as always practice and experiment to find the technique that works for you.