Shut off your pools pump so that no water flows in or out of the lines.
Unscrew the cover from one of the return line vents in the pool wall. The return line vents are usually about half way down the pool wall, and shoot jets of water back into the pool after it as been run through the filter. You should be able to unscrew the vent by hand, but use a wrench or channel locks if you have trouble.
Screw the aerator bracket into the vent. Tighten the bracket as far down as you can. Make sure the bracket is firmly in place before proceeding.
Install the plastic tube or pipe that came with your specific aerator. One end of the tube or pipe should screw into the bracket, and the other end should protrude out of the water.
Insert the loose end of tube or pipe into the above ground mount. Point the mount towards the middle of the pool so the water will land in the center as it is discharged from the aerator.
Turn the pool pump back on so the water begins flowing back through the return lines. Adjust the stream of water shooting out of aerator so that it lands in the center of the pool.
Place a cover over your pool when it is not in use on sunny days. The less sunlight that heats the water, the cooler your pool will stay.