What i do is to loosen my embouchure slightly and make a kind of "uuuuur" noise from my throat down the sax, by the way mouthpieces make it easier/harder as well, i find it most easy on my Floridated 9* link, easy'ish on my Lawon 10b and quite difficult on my newly aquwired Guardala 9*
B.T.W. as far as efects go i found this much easier to establish than say, vibrato, altissimo etc but i know for other people it can be the other way around, good look.
It might be because I am Scottish but I found growling easy, just sing Grrrrr... into the instrument, a bit like clearing your throat. It might be easier if you get your throat/aural chamber in the correct shape for the particular note you are playing. Say play a low G, then sing the same note with the best singing voice you have, really open your throat so you get a nice tone, then keeping the shape, Grrrrrrrrr and play the G.