Unplug the mattress heater and unzip the mattress cover on the soft-side waterbed.
Remove the mattress bladders if they are light enough to lift. Place the bladders in a bathtub. Otherwise, open the first fill valve and attach one end of a garden hose to the valve and the other end to the faucet adapter included in the fill kit.
Attach the adapter to a kitchen faucet, turn the water on and turn the adapter dial to the "drain" position. Drain each bladder and place the empty bladders in a bathtub.
Examine the foam bolsters inside the mattress topper and note their configuration. Remove the bolsters and mattress topper at the same time. Remove the old foundation. Place the new foundation on top of the bed frame.
Replace the mattress topper and situate the heating pad and foam bolsters to create a frame for the mattress bladders. Arrange the bladders in the same position they were originally installed. Attach a garden hose to the first fill valve and fill the bladder 3 inches. Fill each bladder in turn, 3 inches at a time until the water reaches the fill indicator line.
Dry any excess water on the bladders and mattress cover. Zip the cover and plug the heater back in.