Unplug your water bed heater from its power source.
Remove the aerator from the nearest faucet. The aerator is the small piece that screws onto a faucet to control its water flow.
Connect the faucet connector--part of a water bed fill kit--to the faucet.
Connect a hose to the faucet connector.
Connect the other end of the hose to the water bed hose connector, which is also included in water bed fill kits. The water bed hose connector is a long valve that will fit into the fill valve on the mattress.
Remove the cap and plug from the mattress and pull up on the valve until it pops out.
Add water bed conditioner, which will help keep the vinyl mattress from drying out and cracking.
Insert the water bed hose connector into the valve and turn on the faucet.
Add lukewarm warm to the mattress until it's at your desired level. If you are filling it from empty, fill it until the mattress is about a half inch below the top of your water bed's frame.
Replace the cap and plug and push the valve back into the mattress and turn your water bed heater back on.