I made this flag animation by following Andrew Price’s old flag tutorial. I didn’t use a subdivision surface modifier, so the flag may not be animated all too well. The version of Blender that I was using is 2.79, so I guess the old tutorial still works in the current version of blender. 
Btw, you may notice a brown horizon color below the flag; I was using blender’s new feature, dynamic sky, and I don’t know how to change the location/direction of the simulated dynamic sky. Hopefully, I can figure this out soon.