How can I make covers for tall back chairs?

Depends on the look you want. A photo would help.

I've seen cute covers that are just a flat piece of fabric with a ribbon wrapped around the lower part of the back and tied into a bow

Check that pin https://www.pinterest.com/pin/334744184804035799/

This might help! http://aplaceofgratitude.blogspot.com/2013/04/how-to-recover-parson-chair.html?m=1

Pillow cases would work.

Hi Dianna, hope this helps

https://www.youtube.com/watch?v=mYT-z6a_SSY

https://www.youtube.com/watch?v=yV9onlGcXQU This should help.