Unfold the water mattress inside the frame, remove filler cap and bung. Make sure your mattress is placed central in the frame leaving equal material each side and at each end. The mattress will appear to be too large and will overlap on the sides of the frame. Make sure that the mattress corners fit snugly into the corners of the frame. Click here to view Mattresses for a Hard sided waterbed Click here to view Mattresses for a Soft sided waterbed How To Fill The Mattress Place your hose pipe into the filler cap and start filling with cold water. Add approximately 1 inch of water to the mattress then pull all the wrinkles from the bottom of the mattress and once again make sure your mattress is positioned centrally in the frame.  Fill mattress to the top of the frame pulling wrinkles from the bottom of the mattress while doing so. Click here for filling tools Now you can plug in your heater.