To fold a fitted sheet, you must decide on a criterion for what a successful fold looks like for you. I assume if you are asking the question "How do I fold a fitted sheet?" in the first place, you probably care if the sheet looks neat and fits into a drawer but maintains clear boundaries among other stored, folded fabrics, which typically means is rectangular and relatively compact.

So the only thing that matters is that you have a big enough clean surface to use while folding and that you locate and hold the sheet by the corners of the sheet with seams. Your goal is to get the corners lined up so that you can hide the rounded edge of the damn thing inside of itself, to keep the macrostructure rectangular. Once you have a rectangle, you can fold it however you like and it will be as easy as folding a rectangular sheet.

Anything goes at that point. The only thing most people don't get is that you can make the whole thing easy if you start by turning the circular topographical nightmare into a rectangular mass by finding and focusing on the rectangle within it, which is marked by the seams of the corners.

All that tortuous frilly elastic shit that swallows you whole in the middle must be ignored and then hidden by keeping it constrained by the rectangle you can find when you focus on the seams. Remember, ultimately, there are four corners, and the fabric stretched out is just a rectangle.

Now ask yourself one question... Do you control your fitted sheet, or does your fitted sheet control you?