I do sour mashes fairly often, for wits and Kentucky Commons(I live in Louisville). Eventually I'm going to do one for a stout. My method is to take about 20-30% of my pale barley and do a regular sacharification infusion(150 or so) in a small playmate cooler. I wait until the temp drops to 120 and add a little uncrushed grain to the mini mash and cover the surface with plastic wrap. About every twelve hours I'll add an infusion of boiling water to bring the temp back up to 120. Keep the oxygen out with a covering of plastic wrap. I keep this up for 2 or 3 days, depending on how well I've planned ahead and how much sour I want. I'll usually heat it up (maybe using it as a decoction) for mashout. The only problem I've ever had is one of my wits turned out too sour and was more like a berliner weise.
This is a good resource, and what I went by when I first tried to do sour mashing:
Brew Your Own: The How-To Homebrew Beer Magazine - Techniques - Sour Mashing: Techniques
Edit: For the wits or other light beers I'll use the sour mash to bring down the ph