Possibly the most simple to fold of all fitted items are crib mattress pads:
Lay your crib pad out flat with the waterproof side on the bottom. The cotton side should be facing up.
Fold the elastic sides over on all four corners and sides.
Bring the right side of the crib mattress pad over to the left side and match the corners. You’re going to have to smooth it flat and tug it a little to get it straight.
The elastic rim is going to be smoothly tucked on the inside. This is the trick!
Fold the bottom edge up to the top edge.
Turn it lengthwise so it’s easier to fold into thirds. Then bring the bottom 1/3 of the way up. Then complete the fold and smooth out the material.
How to fold a crib sheet might also be helpful.