How do you drain a hot water heater

The first thing you want to do is turn off the hot water heater's power and water sources.

The power can be turned off at the breaker box. You should have a breaker that is labeled especially for the hot water heater.  Flip this breaker off.

Next is the water source. You should see a knob or lever on one of the pipes coming from the wall to the hot water heater. Turn this until it won't turn anymore. If you do not have a lever at the hot water heater, you will need to find the main water shut off for your home and turn the water source off at that point.

Draincock Once the electricity and water source are both off, you will want to look for the draincock. This will look a lot like an outdoor faucet and is located near the bottom of the tank on one side.

Attach one end of a garden hose to the draincock and stretch the other end of the hose so that it will drain outside. If the hose isn't long enough to reach to the outdoors, look for a drain on the floor. Most homes should have a drain very close to the hot water heater. If there is a grid type cover over the drain, then you will want to pop that out, so that the end of the hose will go inside the drain.
After you have the hose situated, you will want to slowly turn the knob of the draincock. You should be able to feel/see the water draining through the hose.

Most sludge and/or debris will be settled on the bottom of the hot water tank, so you will not have to drain the tank completely.

Once you see that the water is coming out of the tank clear and without any sediment, you can turn the draincock off and remove the hose.
Turn the water back on at the hot water heater and make sure the tank is completely full before you turn the electricity back.

Warning:  If you turn the electricity on before the tank has filled up with water, you will burn up your water heater elements (and that is a whole different article)
Other resources:

How to Test The Relief Valve On Your Hot Water Heater

How to Check Circuit Breakers or Fuses in Your Home

How to Inspect Electrical Cords for Damage

How to Inspect & Replace Electrical Receptacles and Switches