It’s a combination of getting so much of the note and a blend of the actual growl. You can play the note on it’s own, then try to do just the growl on it’s own, away from the horn, then on the horn, full out don’t be shy. Now it’s your job to start and learn how to have control of both and start trying to blend them to your liking. Sometimes we want lots of growl and other time just a hint of it will do.