The best way to practice this is to play a clean note, then while holding the note steady, hum any old note in the back of your throat. Alter the pitch of this hummed note until the interference with the saxophone note creates a good growl and hold it there for as long as you can.
Repeat the process with different notes, (if you want to be really disciplined you can go slowly up and down the chromatic scale) doing the same on each note. You will gradually get a feel for which note to sing or hum, and after a while you should be able to do this without thinking about it.
As with any effect, the growl can lose its “effectiveness” if overdone. That isn’t to say there aren’t contexts in which you can usefully use the growl constantly, but these are often kept to short solos in an appropriate genre, e.g. Rock & Roll. It’s useful to be able turn it off and on, or vary the intensity.
Ideally you will learn to control your growl to such an extent that you are able to add just a very very slight amount to your sound. You should be able to do this so instead of being heard as an actual growl, what it does is just give your tone a little bit of edge or brightness.