The best way I can explain it is to say "argggghhhh" in your throat while you play. It takes a while to get used to how you have to do it, but you will get the hang of it. Try making the sound before you play, the playing and making it, thats the best way to get started. It doesn't come easy, but you will eventually be able to do it. Its the easiest in the middle range... the higher and lower notes are tougher to growl on, but if you practice it, you will be able to do it eventually. Good luck with it and BE PATIENT, I have been working on mine for months and it still isn't perfect.

All you have to do is hum into your instrument. It works better when you hum 3 notes above the note your playing. It's kind of hard at first, like patting your head and rubbing your stomach at the same time. Took me about a week to be able to acomplish the growl. Hope it works for you.