Re: How to flutter tongue
Growling is just humming while you play your horn. Don't make if more difficult than it is. You can play around with trying to harmonize notes you sing with notes you play, but generally, this isn't necessary if you are just trying to get that growling tone.
Flutter tonguing is literally fluttering your tongue while you're playing your horn...sort of like rolling a spanish "R" sound, fluttering your tongue against the roof of your mouth.
Generally, growling is a more musical embellishment. It sounds especially good on alto...listen to Phil Woods and Cannonball, they use it very tastefully and to great effect.
Like any effect like this, a little goes a long way.