Hygiene is always important, even with our bedding. If you can't see stains or smell any odor, it doesn't mean your sheets are clean. Sheets can accumulate dead skin, mites, animal dander, pollen, ...

Now, you should have a rectangle-ish shape to work with. Fold the sheet in half so that the elastic edges are hidden, smooth out the corners and fold the sheet in half again. Keep folding it in half ...