I have a fairly extensive and well illustrated description of using acrylic washes, in what are called "Reference pages" on my blog. They are found in the upper right corner of each blog post. Here is a link to Part 1, which is the basics (there is also a Part 2 with more advanced and more detailed aspects):
I continue to rely primarily on this method, with assists from artist's color pencils and Pan-Pastels for added effects.