How can I use a diffuser to make my frizzy hair smooth?

That's what a diffuser is for. It makes it so that not all of the heat from the blow-dryer is in one spot, but spread out. This ensures that your hair will not get frizzy or that there will be a minimal amount of frizz. Just use the diffuser like you would a normal blow-dryer. Wash your hair, apply any product you want to in it, then dry. Try scrunching your hair as you go to help develop the curls as well.