Remove the fuel tank according to the instructions specific to the make and model of your car. Drain the fuel into a fuel caddy. Clean debris from the top, and flush the tank with water. Clean the inside of the tank with a high-pressure mixture of dish soap and water.
Check for rust and road debris near the top of the tank, particularly near the hanger or module assembly to keep contaminants out of the tank. Take off the lock ring and the hanger or module assembly. Flush the tank with water, and install the new hanger or module assembly.
Swish the fuel tank in a circle to gather any debris left in the bottom of the tank. Pour the water and debris into an approved disposal container, invert the tank, and smack the bottom to knock any remaining debris out. Put a mixture of dish soap and water through a high-pressure spray system to get around the baffling that keeps water from easily entering many modern fuel tanks. Circulate air through the tank, or set it outside on a warm sunny day to let it dry.
Wipe as much of the fuel tank out with a lint-free shop towel, and run compressed air through the tank to dry it completely. Look at all parts of the tank to check for rust or signs of damage. If either is present, repair the flaw or replace the tank. Let the tank sit for 30 minutes after drying it with compressed air, and then add the fuel back to it from the caddy, and reattach it to the car.