The first and most critical step is to ensure your safety and rule out the simplest possible cause. Go to your home's main electrical panel (breaker box) and switch off the circuit breaker that supplies power to your water heater. It is often a double-pole breaker. For gas heaters, you would turn the gas control knob on the heater to the 'PILOT' or 'OFF' position. Once power is off, locate the cold water inlet pipe on top of your water heater. There will be a shut-off valve on this line. Check to ensure this valve is fully open. Sometimes these valves can be accidentally knocked or not fully opened after previous work, restricting flow to the entire hot water system. Turn the handle counter-clockwise as far as it will go.

Locate the drain valve at the very bottom of the water heater tank. It will look like a small spigot. Carefully attach one end of a standard garden hose to this valve. Run the other end of the hose to a suitable drainage point, such as a floor drain, a utility sink, or outside to a lawn area that won't be damaged by hot water. Ensure the entire length of the hose is running downhill from the valve to allow gravity to drain the tank effectively. Place a bucket under the valve connection to catch any initial drips when you open it.

With the hose connected, turn OFF the cold water supply valve (the one you checked in step 1). Next, go to a sink inside the house (the kitchen sink is often a good choice) and open the hot water tap fully. This is a crucial step that allows air to enter the plumbing, breaking the vacuum and allowing the tank to drain. Return to the water heater and open the drain valve using a screwdriver or pliers. Water, which will be hot and possibly discolored with sediment, will begin to flow out. To aggressively dislodge stubborn sediment, wait for the flow to slow, then close the drain valve, turn the cold water supply ON for 10-15 seconds to stir up minerals at the bottom, then turn it OFF and re-open the drain valve. Repeat this 'pressure flushing' process 3-4 times until the water draining from the hose runs clear.

Once the water runs clear and the flushing is complete, make sure the drain valve at the bottom of the tank is securely closed. Remove the garden hose. Go back to the sink where you left the hot water tap running and leave it ON. Return to the water heater and slowly open the cold water supply valve. You will hear the tank begin to fill with water, and air will begin to hiss and sputter from the open faucet. This is normal. Wait until a full, steady stream of water is flowing from the faucet with no more air sputtering. This indicates the tank is completely full. Once confirmed, you can turn off the sink tap. Now, carefully inspect the drain valve and all other connections on the water heater for any signs of leaks.

With the tank full and leak-free, it's time to restore heat. Go back to the electrical panel and flip the water heater's circuit breaker back to the 'ON' position. For a gas heater, follow the manufacturer's instructions to relight the pilot light and turn the gas control knob back to its normal operating setting. The water heater will now begin to heat the new tank of water. This can take anywhere from 30 minutes to over an hour, depending on the tank size and heater type. Once sufficient time has passed, go to several faucets in the house (kitchen, bathroom sink, shower) and test the hot water pressure. It should be significantly improved and nearly equal to the cold water pressure.