How do i drain a water heater without making a mess?

Turn off the power (or set a gas one to pilot/off) and shut the cold water supply valve on top. Let it cool for a few hours if you can, tank water is hot enough to scald. Screw a garden hose onto the drain valve at the bottom and run it outside or to a floor drain that's lower than the valve. Open a hot tap somewhere in the house so air can get in, otherwise it drains really slowly. Then open the drain valve. The mess usually comes from the hose fitting dribbling, so keep an old towel and a bucket under it. When it's empty, briefly open the cold supply to flush out sediment, then close the drain, refill with a hot tap open until water runs steadily, and only then turn the power back on. Firing an electric heater with an empty tank burns out the element.