Put your hands into two of the adjacent corners, attached with a piece of elastic.
Place one of the corners inside of the other corner so that the outside, finished edge is exposed. This will create a pocket.
Repeat with the other two corners.
Place your left hand into the left corner pocket and your right hand into the right corner pocket.
Insert the left corner pocket into the right one so that all the elastic corners of the fitted sheet are together. This will create a pocket consisting of all the corners. Smooth out the pocket to create as straight of an edge as possible.
Lay the sheet down and smooth it out so that you have a rough rectangle with four corners, including the corner with all the elastic. Fold down the edges of the sheet to make the lines as even as possible. Keep smoothing and folding as necessary until it is as square as possible.
Fold the sheet like you would regular sheets until it is the desired size you need.