It’s tedious, but it’s mostly just a matter of waiting for the water to heat back up afterwards.

Unplug the heater, then run the bathtub until you’re out of hot water. Turn off the bathtub, and turn off the water supply going into the heater. Attach a garden hose to the bottom of the heater, and run it to a drain somewhere. Turn the hot water on somewhere like a sink, to provide a vent to the system; It won’t run because the supply is turned off, but it’ll at least allow air into the system for when you drain the heater. Use the spigot at the bottom of the heater to drain it.

Be prepared for a gross rush of water at first, because all of the sediment and rust will have settled to the bottom of the heater where the spigot is. Once it’s drained, you can do whatever maintenance you need. For instance, you probably need to replace the cathode rod, which is designed to wear away over time to protect the rest of the heater from corrosion.

To do a final flush, you can open the water inlet at the top of the tank. It’ll help drain any last remaining sediment out of the tank. Then turn off the water inlet, turn off the spigot, disconnect the hose, and leave the sink tap open while you refill the tank with fresh water.

The whole process only takes like 30 minutes, and then you’re just waiting for the heater to come back up to temperature afterwards.