With the exception of using a electric pump, you can achieve this goal with a garden hose and a hose extension ALWAYS UNPLUG THE HEATER ( when filling or draining your water mattress) .
NOTE: If you are draining an all water type mattress. (no fiber). skip steps. (3)...(4)...(5)...
(1)...The hose extension screws onto the male end of your garden hose (a hose extension is made of plastic and is approximately 2" in length) You must have this to fill and drain your mattress. It can be purchased at your local Waterbed store. Make sure the hose extension is submerged into the water.
(2)...Run the hose off the bed onto the floor and down your hallway, out your front or back door. Once outside hook up the female end of your garden hose to your outdoor faucet. Turn the water on full blast, let the water reach the mattress. Once the water has reached the mattress, turn the water off and quickly disconnect the hose from the faucet and lay the hose down. In a moment the water should start backing out of the hose, pulling the water from the mattress behind it. (Your draining experience will go bedder when you work with gravity... Going out a high window with no drop can be a slow process).
(3)...As the mattress drains, the fiber compresses, trapping the water in the mattress. When the mattress is half drained, pull the hose from the mattress and let it breathe for about three minutes. You will see the mattress breathe and the fiber expand, releasing the trapped water.
(4)... Next, repeat the draining process. As the last half of the water is almost drained, go to the head of the bed and grab ahold of the mattress while HOLDING the fiber in place, lift and fold the mattress to force the remaining water toward the filler valve.
(5)... Once the mattress has stopped draining it might require a helping hand to transport the mattress outside or to a bathtub to let the remaining water flow out. A wet-dry shop vac also works well to empty the remaining water when some water is left in the mattress. When using a pump or shop vac always be sure to plug and cap the mattress after vacuum packing, this will hold the fiber in place and keeps the mattress compressed for transport.
(6)...Use caution when transporting the mattress because this is the time when punctures may occur. This method works well for most of us but for those who cannot use a garden hose outside, you can purchase an inexpensive plastic pump that attaches to your kitchen or bathroom faucet. You can also rent an electric pump from your local rental store that will do a fast job.