Per ATWOOD:
1. Turn off your main water supply. Drain you water heater tank. Reinstall drain plug. Remove the pressure-temperature relief valve. With a funnel use 4 parts white vinegar to two parts water. (In a 6 gallon tank that would be 4 gallons vinegar to 2 gallons water).
2. Cycle the water heater, letting it run under normal operation 4-5 times.
At no time do you remove the vinegar from the tank Once this has been completed, remove the drain plug and drain the water heater.
3. After thoroughly draining the tank, to remove the sediment, flush the water heater.