Remove the cap and plug from your empty waterbed tube.
Lay the tube flat in your bathtub with the tube's fill valve facing the tub's faucet.
Fill the tube with lukewarm water to the firmness level line that you desire.
Push any air bubbles that formed while filling the tube out of the fill valve.
Put the cap and plug back into the tube.
Wipe any water that spilled during the fill process to avoid mildew forming on the tube when it is placed in your mattress.
Place the filled tube in your waterbed with the cap at the foot of the bed.
Repeat the process until all of your tubes are filled to your desired firmness level.