start here by smoothing out your sheet
Lay the fitted sheet on a flat surface, with 2 corner seams at top & bottom
( A B at the top and C D at the bottom )
Turn two bottom corners (C & D) inside out and tuck corner C under A,
then D under B. Smooth, making a flat rectangle shape
Fold the fitted sheet in half from right side meeting in the middle.
Repeat with left side to form a long rectangle.
Fold in half again and once more if needed to suit storage space.