Pair the corners
Hold the sheet lengthwise with a hand in each of two adjacent corners. Bring the corners together and tuck one over the other so the seams line up. Repeat with the other pair.
Make one nested set
Bring the two paired sets together and tuck them into each other. Place the sheet on a clear surface. Arrange the elastic edges toward the inside; aim for a rough rectangle rather than perfect straightness.
Fold to fit the shelf
Fold the rectangle into thirds lengthwise, then fold it across two or three times. Use the shelf depth as your guide. Store the matching pillowcases with the sheet so you can retrieve a whole set at once.