First, you let your horn fill up with spittle. . . .Pipster said:
It's not flutter-tonguing. It's singing or humming into the horn while you play. Works best from D2 up.
(Not growling, singing or humming. You can also play around with varying the pitch to see what works best. If you have a good ear for it, harmonies are possible, though weird.)
If you use the "search" feature you can find a good deal of information about this and other effects and techniques; check out John Laughter's material in particular.