Hi, I have not actually used a "wash", but I too am intersted in it. I found a good site that discusses the different types of washes and how to apply it. It actually sounds really simple. Go to:
http://www.aircraftresourcecenter.com/
Then click on "tools n tips" at the top of the page, then scroll down and click on "washes & weathering". The tips are there.
Also, as you can see, this site has tips for basically everything. I read through most of them and learned a lot. Quick point I pickeed up is that if you are working with clear parts, dip them in Future floor polish and after it dries it looks like glass. I do it for all my aircraft canopies now.
Hope this helps.
Paul