Wash and dry your sheet completely. Iron to remove any wrinkles. Place your sheet on a hard, flat surface.
Fold the sheet in half lengthwise with the right sides together. Cut two rectangles from the king sheet, each measuring 78 inches long and 38 inches wide.
Pin the rectangles together on both long edges and one short edge. Sew around these three edges using a straight seam and a 1/2-inch seam allowance. Back stitch at the beginning and end of your seams.
Fold in the remaining short edge a 1/2-inch and iron to keep in place. Fold in another 1/2-inch and iron and pin to keep in place. Sew around the entire top edge with a straight seam and a 1/4-inch seam allowance.
Pin one piece of the hook-and-loop tape 2 inches away from the edge of the open end on the right side of the fabric. Pin the other piece of hook-and-loop tape 2 inches away from the edge of the open end on the wrong side of the fabric. Sew each piece into place on all four sides of the tape using a straight seam and a 1/8-inch seam allowance.
Turn your fabric right-side-out. Feed your foam pieces into the cover, one at a time, so they are side-by-side. Secure them in place by closing the ends of the cover with the hook-and-loop tape.
Fold into a chair by placing one piece of foam on the ground with the rest of the pieces to the right. Flip the next piece of foam up on top of the first piece by folding it over to the left side. Make sure there is ample fabric between the two pieces and that it does not pull or pucker the foam. Fold the remaining piece of foam to the right, on top of the other two pieces.