I use sta-flo diluted 50-50 with water and bake all my bows (Don't have any purchased instructions, just learned by word of mouth & trial & error).
Never tried the microwave, but have had a fellow loopie recommend it for korkers. She says it works. If you are going to try to microwave ribbon, make sure it is saturated with starch first & ensure there isn't any metal on the bow
I think cornstarch might leave a chalky, white film on the bow. Some people use it in mineral makeup. When I tried it, my face looked chalky white lol (and I'm already pretty pasty!)