Remove the cushions from the sectional. Remove the fabric cover from the cushion by cutting the seam open with the seam ripper. Use caution to prevent damaging the cushion.
Measure each cushion to determine the length, width and height. Write each measurement on a piece of paper. Add the height and width together and multiply the number by two. Add 2 inches for the new measurement for seam allowance.
Calculate the height/length of the cushion by adding the measurement numbers together. Add 4 inches to the new measurement for seam allowance.
Unfold the new covering fabric and place it on a flat surface. Draw the multiplied height/width and height/length measurement on the fabric with an erasable marking pencil. Cut out the marked covering piece with the scissors.
Fold the cover piece in half along the width so the right side is to the inside. Pin the length sides together, and do the same with one of the short sides. Sew the seam at 1/2 inch. Turn the sewn cover so the right side is out.
Fold under 1/2 inch of fabric to the wrong side on the open edge. Iron the fold to hold the fabric in place. Slide the cushion into the covering like it is a pillowcase. Sew the covering closed by hand; use a slipstitch to make a clean closure.
Repeat this process to cover each cushion on the sectional sofa.