Breathe in deep, but not so deep that it "catches." You know when you take a MASSIVE breath and then you find you've lost the ability to let out a steady stream of air from start to finish once you go to exhale? You just let out a big puff all at once before you can finally control the stream of air again. You need to be aiming to breathe in a bit less than that, there's a line you can toe, it's tricky to find but extremely important that you do so.
For some of the longer growls on the STC EP (the one in Race of Roses that goes "Crown Of Thoooooooo- *cue hold for quite a few bars*" in particular is a good example,) I achieved it by... Well I won't say "clenching" my throat, but by applying some form of tightness there, some form of tension. Many will say that's not a good idea "WAHHH YOU'LL LOSE YOUR VOICE" but metal vocals are never going to be that great for your vocal chords without knowing when to stop and rest, and as long as you know your OWN LIMITS and know when to stop and rest, your body will adapt and your endurance will increase. I used to be able to do 6 hour days of practicing at college for up to 3-4 hours, then go do a show that night, soundcheck and then play a half hour set in which I pushed myself even harder and held my screams for ridiculous amounts of time. After that, I wouldn't scream for the next day or so to rest my vocal chords then I'd be at it again the day after that.
But anyway, I digress. What I'm getting at here is you need to learn to use your diaphragm and your vocal chords and muscles in that region to regulate your air flow. This is important for shows too. For the STC EP I was literally yelling for most of it, it was less a controlled growl and more of a full blown vitriolic, pissed off scream. This means I was tracking perhaps a line at a time, but live I can get a very SIMILAR sound (not exactly the same, thus the choice of the differing technique on the studio recording) by exerting less force, and applying some form of pressure or tightness in my throat/neck/vocal chords and well practiced air flow using my diaphragm.
A good exercise here is to take a deep breath then exhale for as long as possible with a consistent air flow.
You will probably gravitate towards pursing your lips and feeling some form of tension or tightness in your chest and throat, this is no accident. Apply this to your vocals and you will make extremely quick progress. Compare this to trying to control your air flow with a wide open mouth. The shape of your mouth dictates a lot of your tone and air flow. There's a reason the guy's from The Faceless and Whitechapel and JFAC have that specifically deep and fat tone while still having ridiculously regimented air flow and lyrical articulation, and it's down to the shape of their mouth, which is one of the other key factors of all of this, and is definitely something I took on board for my own vocals.
Anyway, sorry for the essay, but for such a simple question I feel there is a hell of a lot of ground to cover.
edit: and one more thing. Randy Blythe has one of the most potent cases of a voice, lungs and vocal chords totally fucked by smoking and drinking, which means he's probably having to put a whole lot more effort and practice in than either of us are and still laying both of us to waste. If that isn't a testament to the man's ability, I don't know what is. Putting yourself up to Blythe's standards will make you feel like you aren't making progress, I know from experience as his vocals were what inspired me to learn this whole screaming lark in the first place. Keep in mind the guy's one of the most professional in the business, don't beat yourself up too hard trying to emulate him.
