Souring

How does one go about souring? I know how with an AG batch, but what about partial mashes, or extracts? Is souring even an option then?

You can put a few ounces of crushed 2-row in something such as a thermos and add 150-155F water and let it sit for a couple days. Overnight may even be enough depending on how much souring you want of the mash. You can also pull of 18-20oz of fermented wort and innoculate it with brett, but that would take several months for the brett to ferment out, not to mention you would have to pasteurize it before returning it.

If you just want sour, you could also just steep some acidulated malt with your other grains or PM.

If you're going after a sour stout (like guinness) you can just get a bottle of it and put it in a pot with a bit of 2-row and let it sit for a couple days. Then boil it and add it to secondary- thus controlling how sour you want the final beer by adding to taste.

Or something...

Or add a tiny amount of live yogurt to some of the batch & let it sit.