You can look up how to culture it in the academic research but not only is it relatively hard, culturing Bt isn't enough to turn it into an effective pesticide. Bt is a soil microbe and sticking it on a regular agar plate guarantees it will be outcompeted by contaminants so culturing it requires sterile lab conditions and expensive lab equipment. You're going to need a full blown fume hood with active ventilation just to handle the initial samples and that's the bare minimum. Chances are you'll need a sterilized glove box and commercial off the shelf bioreactor.

The end product is produced in large submersion fermentation tanks where the bacteria is first grown, then then forced to sporulate and allowed to ferment which produces the Cry/Cyt toxin crystals within the spores. Going through these phases requires precise control over nutrients, pH, agitation, and electron acceptors. Even activating the spores to create the inoculum and begin the reproductive cycle is nontrivial because of how sterile everything needs to be.