I remember researching this last year when I was going to open my pool and didn't find an easy answer. I would like to share what I did this year and if its already been posted somewhere I apologize. We all know its easy to stick a shop vac hose in down the skimmer hole because the water level is below that but my returns are always under water and have 1 1/2 inch screw in plugs with O rings. I rigged up a 1 1/2 inch PVC male threaded piece with a reducer with a threaded female piece, then a male threaded 90 degree 1" nipple and a 6 foot 1" rubber hose. After I put these pieces together I needed a temporary plug for the 1" nipple so I rigged one with a piece of pipe fitting wrapped in electrical tape anything will work. I then reached in the water and unscrewed the plug and quickly screwed in my PVC rig without the hose attached. Then I pulled out the plug and pushed the hose on the nipple. I handed the hose to my wife who held it over a pail and used the same method as blowing out the lines with that I used to winterize and emptied all 3 returns. It took about 13 gallons out which I overkill in the winter but it gets darn cold in Iowa!