Hehehe...yep, I used to make it yrs ago before I knew it was available on the market (or maybe it was before it was made available).
Gather some worms(caterpillar)...the easiest to use are cabbage worms and loopers. You want to look for the ones that actually look sick. They'll look sometimes chalky white, sometimes they'll be hanging down from the bottom side of the leaves. (They'll eventually turn black and get mushy.)
Collect some, liquify, and make a spray of them. (Half cup of bugs, add two cups water and blend/liquify. Strain thru cheesecloth or a good strainer to get the chunks out. This is your base. When you use it you should cut it with 4 to 8 parts of water. Can also store the base by freezing.)
Sometimes this is considered "nuclear polyhedrosis virus" and not necessarily Bt. However, once the caterpillars are sprayed with a commercial Bt I don't see why you can't collect them and use them as your "starter" for a definite source of Bt. (All this now keeping in mind that there are many forms of Bt now available.)
Would this make ALL the butterflies in your garden die? Except for cabbage butterflies, which are SO destructive, I really enjoy watching the butterflies. Moths, nothing but pests, but butterflies...swallowtails, coppers, blues, monarchs, calif. sisters...in your smoothie??