This SHOULD simple. Somewhere towards the bottom of the water heater there should be a drain valve. Put a bucket under the drain valve, open it and drain some water. start with just a bucket full and if you are getting a lot of crud in the water, drain another bucket full. I do my hot water heater every couple of months. A word of caution, old shutoff valves sometimes won't close all the way if they are old, have not been open/closed in a while, or have corrision built up on it. I have learned the hard way that if it looks like the valve is old and may not open/close easily, don't mess with it and by all means never force a shutoff valve open/close.