Play a D in the middle of the staff on the alto saxophone and hold the pitch out. This will be your drone pitch that you set against another pitch to growl with.
Sing a pitch, while simultaneously playing, that is very close to the D while playing. This takes some practice, which is why you are starting on a lower, easier note. Avoid the tendency to hum instead of sing. Humming will not provide the strength of tone necessary to create a growl.
Experiment with the pitch of the sung tone until you get a nice clash between the two notes that sounds like a growl. If you know how to sing and you have a good ear, try and sing a half step above the note played. If not, use your judgment and listen to the tone to decide what pitches work best.
Growl on a high D to get used to growling in the higher registers. This is more complicated since you have to use your falsetto register if you are a man to sing this high. If you are a woman, you will be using your whistle register. To reach this register you must practice singing with a small amount of air in the highest register. This will make the vocal cords vibrate only on the outside edges and produce a higher pitch.