It's as easy as attaching a hose and opening a valve.
Mine was surprisingly clean considering I have never cleaned it and the heater must be >10 y/o. The water was just slightly cloudy for about 45 seconds. I let it run for about 1.5 min. The reason for the clear water might be we had soft water for about 7 of those years. I've since bypassed the water softener but the water still runs though a charcoal filter.
There are other videos that suggest shutting off the gas, let the water cool, close the intake and opening the relief valve first.
I think I'll just take the KISS route.