Whether you're talking about growly sounds, extreme vocals, extended range singing, distortion on the voice, husky sounds, or even screaming, there are a few things to bear in mind to make sure you're playing it safe when exploring with your voice.
1. Make sure you have no tension
We always come back to this point when we are talking about singing, particularly with regards to exploring new territories with your voice. When you're approaching singing in these more extreme styles, make sure you're not straining, and make sure you're not tensing up any parts of your body.
2. Have patience!
You might find that you want these changes to come over night, or you want to be able to sing in these styles over night, but that's just not going to happen! Forcing yourself is only going to result in injury, and possibly long-term damage. With any kind of exercise growth is not immediate. You are going to need to practice and practice and practice, and be really patient! It will be a slow process, but it will be worth it when you get there.
3. Have self-control
When you hear these really loud, extreme, agressive vocals, your instinct will be to try to recreate them by shouting or putting a lot of aggression and volume into your sound. Don't forget that most of the time with extreme vocals, you're hearing the sound through a PA system at a gig, or you're listening to a recording on an album where the sound has been altered in post-production. This isn't to discredit people who can sing heavy vocals, but more often than not you will be very surprised at the sound you hear without the modifications of post-production or the PA at the venue doing a lot of the work!It's a lot quieter than you'd expect. No matter what you are singing - extreme vocals or not - everything should feel comfortable and safe. If you get on stage and you can't talk, you're doing it wrong.
Good luck exploring heavy vocals, but make sure you play it safe! Long-term damage definitely doesn't sound good.