When I've made bread from beer yeast, I've made a starter culture from the ale yeast. As I recall, I used the goodies at the bottom of a 22 oz. size bottle of one of the Unibroue, or perhaps it was Allagash Belgians (about 1/2c of liquid as I recall), with a pinch or two of sugar and warm (70-80F) water, put it all in a sterilized mason jar with enough bread flour to make a batter, cover loosely with plastic wrap, and let it sit in a warm kitchen until it starts to show signs of life. It's a similar process used to get a sourdough culture restarted. You have to tend to it over the course of a few days, until you've got about 2 c. of "starter" (which sort of resembles a paste). It should smell sour, but with a good fruity / yeasty smell to it.
Unfortunately I'm so used to doing sourdough starters now that I just do it by habit more than recipe. Oddly enough, breadmaking seems very similar to homebrewing - patience and experimentation go a long way towards making edible happiness.