There should be a silver or brass colored drain cock at the bottom of the water heater tank with a threaded end. Hook a water hose to that and run it to a floor drain or, outside.
Open the pressure relief valve (T&P) Flip the lever into an up position. Completely drain the tank, which can be a slow process.
To flush the sediment, turn on the water supply inlet at the top of the tank for twenty seconds or so and let it stir up the sediment. Repeat the process a few times allowing it to drain. But..
On my heater I actually raked the solidified chunks of sediment out of the lower element base hole first. If you have as much in it as he claims it may be necessary. Then do the standard cold water flush to remove the rest.
This process needs to be done every couple of years regardless just to extend tank life. The stuff is really corrosive.