How do you make safe organic fire ant killer?

There is not really a need to go to the trouble. There are many highly effective (although kind of expensive) fire ant baits readily available. The poison in them is completely organic (boric acid) most cases and in minuscule amounts. Ants have a unique feeding process whereby the adults feed the larvae and the larvae liquefy and feed them, so is highly effective if you find one (bait) they will take and no danger to anything secondary. The colonies of course are easy to find since there are always mounds due to design of their back legs. You might also look at a product called termidor and for a contact spray ECOPCOACU (acronym) and also completely organic but extremely powerful.