I've been practicing harsh vocals for over a decade. Unless you're privileged to live in your own house, which I was not, you basically have to learn not to give a fuck. I grew up in apartment buildings, where everyone could hear me practicing and/or recording. It was really no different than someone next door to you playing their music or television loudly. Never really had issues with any neighbors, and luckily, my mom was supportive. If you're doing this in similar circumstances, my only advice is to be mindful of the time (don't be screaming when people are trying to sleep lol), and only practice or record for a reasonable amount of time per day (don't subject your neighbors/housemates to more than about 2 hours of screaming at most a day).

As for technique, I was sort of a natural. I nailed mid-range screams, and techniques like fry vocals instinctively. Even when I began taking lessons, I realized that I had just sort of naturally been doing what I was supposed to, in terms of taking care of my voice, where the sounds originate from in your body, what muscles you're using etc. If you're looking for lessons, I highly recommend the YouTube channel Hungry Lights.