How do you heat an above ground pool cheaply?

You can buy a plastic solar cover. Sort of looks like bubble wrap but thicker and heavier. You can cut it to size to float on the top and catch the suns rays. I don't think they are too expensive.

Purchase a solar cover. It stays on until you are ready to swim and traps the sun's heat in the water.

The other year we bought plastic colorful fish that do the same thing as K Rupp stated. Got them at Leslie's pool supply, probably carried by others.

We have a solar cover for ours - plus a solar heater with pump. Not diy but it works.

You can purchase some car radiators,( ones that are very clean in side), and paint them black. Then plumb then together . Attache the filter pump to one inlet and the other outlet to the hose going into the pool. Position radiators to face the sun. One nice solar heater.

https://www.instructables.com/id/DIY-Solar-Pool-Heater/

http://www.silent9.com/blog/archives/107-DIY-Solar-Pool-Heater.html?/archives/107-DIY-Solar-Pool-Heater.html

https://www.youtube.com/watch?v=HTbNKufpOq8

https://www.youtube.com/watch?v=ylnoHcgploc