Wrap the tape measure around the middle of the cushion to measure the distance around the cushion. Multiply this measurement by .7. Add two inches for hems. If your pillow is 50 inches around, multiply 50 inches by .7 to get 35 inches. Add 2 inches to get 37 inches.
Cut a square of fabric with sides the size you calculated.
Iron down 1 inch on all four sides of the square of fabric. This border will keep the fabric from unraveling when it is pinned.
Lay the fabric right-side-down on a large surface, with one corner at the top. Center the pillow on the fabric with an edge at the top. The fabric will be diagonal to the pillow.
Fold the top corner of the fabric down over the pillow and the bottom corner up over the pillow. Pin the corners together where they meet in the center, using a safety pin.
Fold the side corners in, and pin them together in the same way.
Use additional safety pins to pin the edges of the fabric together from the center to each corner. Pull the fabric taut as you pin. The last pin should be on the bottom of the pillow, 1 inch from the corner.