Strengthen your lungs by performing daily breathing exercises. Take deep breaths through your nose, holding the breath in your lungs for 10 to 20 seconds. Exhale slowly through your mouth until your lungs are completely empty. Repeat this process for five minutes, adding an additional minute to each successive exercise session until you reach 30 minutes of daily training.
Listen to as much sludge metal as possible, paying attention to the vocal range of the lead singers. Take note of the most common vocal pitch used during the songs. Sing along after repeated listening sessions using a very calm and quiet voice. Learn the phrasing and rhythm used in the songs without attempting to match the volume and power of the vocals.
Drink warm water or tea prior to singing at full volume. This loosens up the vocal cords and opens your throat, enabling you to create deeper and more powerful sounds.
Sing along to the same songs that you practiced in Step 2, taking breaks as needed between songs. This time, however, increase the volume and power of your voice by pushing air from deep within your diaphragm. Tighten your throat as the air passes through to create a growling sound. Keep the overall pitch of your voice low by focusing on creating sounds from your throat rather than your nose. If you feel any pain or discomfort during this process, decrease your overall singing volume until the discomfort subsides.
Wait for 24 to 48 hours before repeating the steps above. Your ability to sing louder and with more control will increase over time, but straining your voice by singing too often can result in damage to your vocal cords. Start out by practicing for a few minutes each day. Gradually increase the length of your practice sessions every other day until you are able to practice for 20 to 30 minutes at a time.
Continue singing along to sludge metal songs, increasing the volume and power of your voice as you become more comfortable. After several weeks of preparation, your lungs, throat, and vocal cords will be conditioned enough to attempt an extended performance at full power. The total amount of preparation required varies from person to person due to age and health factors.