Mash as usual. Do NOT try to sour the mash before mashing -- it will get sour but will not convert. Amylase seems not to work at low pH.
Take the mash to 170F for mash-out. At this point it contains no lactobacillus, so we will have to introduce some. There are four obvious sources: yogurt, sourdough, and "wild" lacto from grain hulls or the air. You need to cool the mash to the correct temperature for your lacto source.
In all cases, keep the mash at your fermentation temp until it is ready. That will take one to two days for yogurt culture, maybe three days for sourdough. The only time I tried wild lacto, it was like lightning -- five or six hours.
When the stuff is done, it will look and smell spoiled. There is nothing uglier than a lactic fermentation, and your marriage may be in jeopardy from the stench. The mash will look soupy, with husks floating on top and a lot of bubbles. If you can get it past your nose, you will find that the liquid tastes good. Sour, but good.
I suppose you can let the mash ferment to completion, but I don't. It would be terribly sour. I go for my target pH, and then raise the mash back to 170 degF for sparging.
Before re-heating your mash, you should pull some out -- say a half pound to a pound -- and keep it. That way, if you like your sourmash beer, you have a ce ready-made for your next batch. It will keep for a surprisingly long time in the refrigerator, with a lid on it. Feed it every month or so. You can even make sourdough bread out of it.