Rent or purchase an electric pump. Waterbed stores and some retail stores carry these pumps, or you can rent one from an equipment store or waterbed dealer in your local area.
Remove the connector with threads on both ends from the waterbed fill and drain kit. Pull the mattress valve stem out of the mattress. The valve is located at the bottom end of the bed. Unwind the cap from the valve stem. Unplug the waterbed heater.
Place a towel around the valve stem. Place the side rail at the head of the mattress and slide the rail from the head of the mattress down to the valve stem to remove any air bubbles.
Wind the connector onto the valve stem. Attach the hose that comes with the electric pump to the connector on the valve stem. Connect a garden hose to the outlet on the electric pump. Plug the pump into a wall outlet.
Hang the garden hose out the window or, if you don't have access to a window, run the hose to a bathtub. Turn on the pump. The water will begin pumping out of the mattress and into the bathtub or out the window.
Hold the connector on the waterbed valve stem down so it is continuously sucking water out of the bed and not air. If the pump is running and no water is being sucked out of the mattress, the pump will burn out.