Best bet is to find an instructional video by Primos or MAD calls. It really is a simple as blowing through them, but there are times to do it loudly and long and there are times to do it softly and/or short.

There's a guy named Pete Fiducia who has some videos out too. He's a little out there, but he goes into a lot of detail about how and when to use calls that's pretty helpful.