it's as easy as mashing a small portion of the mash a day or two before brew day. once the mini-mash is mashed and cooled to about 100F, a handful of fresh malt is stirred in and the bacteria which cover the malt is allowed to reproduce for that time before the main mash. (it's best to keep air away, and maintain the temp. for the duration of the mini-mash) on brew day, you can mix the 'soured mash' right in with the main mash. there are plenty of other ways, but this, i find is the simplest.