How do you make fire extinguisher powder?

There are two kinds of fire extinguisher powders.

The first is monoammonium phosphate, which you find in A-B-C dry chemical extinguishers. To make it, add phosphoric acid to ammonia water until the mixture turns acidic, and the monoammonium phosphate will crystallize out of solution.

In B-C fire extinguishers you will find sodium bicarbonate. This is baking soda, and you can get that at any grocery store.