I did that for a Berliner Weisse. I made a 2 quart starter with a handful of crushed grain. I kept it between 100-120F for 3 days. In the article he mentions that when the pH gets below 4.3 the bad bugs won't grow, so I just acidfied it to pH4 to start with. Then I pitched it along with a vial of lactobacillus and then the next day German ale yeast.