One way to do that is by the "kettle souring" process where you pitch lactobacillus and hold the kettle between 90-100°F for 36-48 hours (you also want to keep oxygen out if possible), or until the pH drops to where you want it. Then you boil the wort and add some hops and/or fruit if you'd like, then ferment with a low pH tolerant yeast. Boiling kills the lacto (so will the hops), so you don't have to worry about contaminating your equipment.
This new yeast @jmck linked above can skip the kettle souring process, because the yeast itself produces the lactic acid as a byproduct. So it does the souring and fermenting all in one. I have yet to try it, but I know it works!