When I make homemade starch, I put the water in a saucepan on the stove. When the water is boiling, I add the cornstarch (mixed with a bit of cold water) to the hot water and wisk. I let it cool and put it in my spray bottle. Homemade starch does not keep longer then 2 weeks for me. I have tried adding lemon juice and alcohol but it still forments eventually.
Sta-flo is the better alternative if you can get it.