I once had a woman at a laundromat ask if she could pay me to fold her fitted sheets. I politely declined and taught her instead.

Stick your hands inside the “pockets” that the corner seams form, starting with two adjacent corners. Match up the points, and then flip one corner over onto the other, so that both corners are now over one hand. Set this side down and repeat on the other side.

Then repeat with the two “double stacked” corners, so that all four corners are layered over each other. At that point, you should be able to fold the “side” parts to form a normal rectangle that’s about 1/4 the size of the mattress. Then fold that rectangle normally.