Remove any remaining connections from your tank (fuel lines, sensors or vent covers). Put duct tape over all the openings but the main fill hole.
Mix the water and muriatic acid in a glass or plastic container. You do not need much: about 1/4 gallon for a small tank, or 1 gallon for a tank with a capacity greater than 4 gallons. Use a 5 to 1 ration of water to acid.
Pour the mixture into the tank, and cover the fill hole with duct tape.
Shake small tanks for three to five minutes to activate the acid wash, then remove the tape from the fill hole and drain the wash from the tank back into your container. Immediately flush the tank with running water for 10 minutes. For larger tanks, pour in the acid mix, seal the fill hole and roll the tank on the ground. Rock the tank from end to end as well. Do this for five minutes, uncover the fill hole and drain the acid. Immediately flush the tank with running water.
Remove the tape from all the openings. Let the tank air-dry for a minimum of 12 hours. Or set a blow-dryer on low, and aim it through the main fill line. Drying a tank with a blow-dryer will speed the process to one or two hours (depending on the size of the tank).
Tape all the openings closed except the main fill hole. Pour in your liquid tank liner (use the amount recommended for your size tank by the manufacturer), and reseal the fill hole. Rock, but do not shake, your gas tank to move the liner over the interior. For larger tanks, you should also roll them on the ground to ensure an even coating. Do this for 10 minutes. Then drain the leftover liner from the tank.
Remove all the tape from the openings on the tank, and set the tank aside to air-dry for 12 hours. Do not use a blow-dryer; the force of the air could strip the liner from the interior or cause it to form puddles.