I just did this with a whole sixer's worth of Bell's Pale Ale dregs. I used about 470ml of water to 1/2 cup of DME, and after a few days (in a flask on a stirplate, with foil), added a second batch of the same. I then chilled that for a couple of days, decanted the old liquid, and did the same thing a second time the next week. I did so because after chilling the starter the layer of yeast didn't seem quite enough. Good Luck!