How do you cover an above ground pool?

Because your question is very general, so will then be the answer. If you would like more specifics, then please ask a more specific question.
You basically need to protect the pool from freeze damage.

Start by cleaning the pool of all large debris, Lower the water level of the pool. install a plug in the return and an aquador over the skimmer. Test and balance pool water.
Install air pillow, cover with cable and ratchet. Make sure all the water is out of the pump, filter, heater and feeder if they will not be put away to protect from freeze damage.