How do you drain an American Water Heater FG series water heater?

Move the gas valve to pilot.
Close the stop valve on the cold water input line,
Open a hot side of at least one faucet,
Attach a hose to the drain valve,
Open the drain valve.

Why are you draining draining the water heater?

I like to start the water heater draining process a little differently. First make sure to turn off the source of power. Connect a hose to the drain valve near the bottom of the tank, and open the valve - full pressure will help move any accumulated sediment / scale through the narrow port of the drain valve. Let the tank drain / flush under pressure for 5 minutes or so, and then, close the valve on the cold water supply pipe to the water heater, usually it is located on the cold water pipe directly above the tank. Now, open the handle on the safety relief valve; this will allow air into the tank preventing a vacuum from forming, and, will help the water flow at the drain valve. MORE importantly, the temperature & pressure relief valve should be exercised annually to make sure it is in good working condition.
Make sure to fill the water heater before turning the power source back on - I like to let the filled tank discharge through the open safety relief valve into a pail; this will insure a filled tank, and, when I let go of the relief valves handle, I'll know if it will shut off properly........

And, never drain a warm / hot water heater into a toilet........