Remove the cap and plug from the tube.
Lay the tube flat in your bathtub and fill it with lukewarm water. Fill the tube to the line that marks your desired firmness--lines will note where to fill for a firm or soft feel.
Use your hands to push any air bubbles from the tube.
Put the cap and plug back into the tube.
Wipe up any water that spilled onto the tube while you were filling it. This maintenance will keep mildew from forming on the tube.
Move the tube to your waterbed and place it in the bed's foam shell. Place the tube lengthwise in the shell with the cap at the foot of the bed.
Repeat the steps until all of your waterbed tubes are filled.