I too am a Stay Flo addict. I love that I can custom mix the strenght, depending the project. Love the crispness and the accuracy it brings to the cutting. Before I discovered Stay Flo I use spray... it got to be more money than I needed to spend on starch.
I let it soak in completely and if I have time let it dry completly for the best results.
When working with scraps I will mix a batch put them (scraps) into a big tupperware bowl , hang on a drying rack iron then cut.
It also makes paper piecing go easier , as the fabrics don't flip back.