Step1
Alright time for step one. First off, you need to realize that your main goal in this is not to strain yourself, but to release anger. That is what Metal/Darkwave is all about. You also need to take into account that if you smoke, this will make the process significantly easier. (Note: It's NOT healthy to smoke, I am not encouraging it.)
Step2
Time to get started! Enter into a quiet room, away from everyone (If you live in the country, go outside and try it there.) Now close your eyes, breathe in VERY deeply, and exhale. Repeat this for about 5 minutes, each time, growling a little harsher, and focusing on anything that makes you angry, or ticked; then open your eyes slowly. Doing this gives your lungs a chance to release any extra toxins or carbon from them.
Step3
When you feel good, insert one ear plug into the left ear, and begin the lowest growl you got to, while working your way up to the inhale/exhale at step 2. Do this until you are comfortable with the sound you are producing, and do NOT take a break until you have the range down, and you can walk (vocally) from extremely low growling, to very soft breathing.
Step4
Repeat step 3 with the ear plug in the right ear, focusing on the sound you are producing.
Step5
Now, time to get edgy. That growl that you are producing is called "Backing" or "False" vocals; which can work well if you have a loud mic, and a small audience. However, if you want it down for good, you'll have to work on it. Next we'll talk about "Vocal Strength."
Step6
Vocal strength is the sound produced from extremely low growling, and screaming (NOT YELLING). To do this, we are going to do alot of what we saw in step two, HOWEVER we will not be focused on breathing to growling, we will begin growling to screaming. Ready?
Step7
Close your eyes, and begin the breathing in step two. Do this about 5 minutes again. Then take a HUGE breath of air, and growl until you feel the vocals straining. This is called your "limit point." DO NOT go past your limit point, if you want to scream (sing) for long periods of time.
Step8
Now, take another HUGE breath of air, and scream at the top of your lungs. Continue screaming until your vocals hurt (note: there might possibly be saliva and flem coming from your mouth, that is good.) Then take a deep breath, and do your vocal scales. Do this for about 10 minutes.
Step9
Now your vocals are all ready for use, you WILL notice that your voice is significantly deeper, and when you try growling, it doesn't affect you as much. Now close your eyes, and start from your lowest growl that you can manage comfortably, shape your mouth into the letter "O", and repeat the word "Yeah" going from high notes to devilishly low notes.
Step10
Now, walk the entire scale of vocalism from breathing to screaming, and you have it down!