On this page is some clear instruction of how to do vocal fry. It's really quite easy once you understand what vocal fry is, and what it sounds like. Vocal fry can be very useful to:
Vocal fry is the initial vibration of the vocal chords. If you send a small about of air to your chords, they begin to vibrate slowly. If you increased this air pressure a little more, you would get a full tone.
So obviously it doesn't take much air to do vocal fry.
Vocal fry is the first sound you make when you wake up. You know the one...
The sound is really gravelly, not quite your full tone. Technically speaking, vocal fry is the lowest part of your singing range.
You can do vocal fry by singing a low note, and then lowering it as much as you can. Soon you will reach a point where you can't sustain a full tone, and your voice will go into vocal fry.
A classic example of it being used, is the cartoon character Elma Fudd. Remember the way he sounds when he says "Be wery wery quite. I'm hunting wabbit". The crackly sound in his voice is vocal fry.
I'm pretty sure that you have already used the above examples to already do vocal fry yourself. But just in case... here is how to do vocal fry...
Say the word "Ahhhh" using as little air as possible. When I say "as little" i really mean it. You should barely hear any noise at all. But the noise you do hear...
...... will be vocal fry!
So there it is. Of course once you know what it is, and why it happens, you should be able to apply it to your singing. Add a little buzz to your more aggressive songs. Use it to scream! Add little interesting elements of style to your songs.