Hold a piece of paper on the front of the armrest and trace the outline for a pattern that fits the curve over the top of the couch arm. Extend the sides of the pattern to a length of 6 to 8 inches, and add 1/2 inch around the entire perimeter of the pattern for seam allowances. Cut a piece of fabric using the pattern for the front of the cover.
Measure the length from the bottom of one side of the pattern, over the top curve and down the opposite side with a flexible measuring tape to determine the width of fabric for the fabric cover.
Determine the length of the armrest cover by measuring from the front of the arm to about 4 inches from the couch back. Cut the arm cover out of the fabric using the width measurements from the front cover and the length measurement of the arm.
Pin one width side of the arm cover to the front cover so the right sides of the fabric are together. Sew the two pieces of fabric together with a 1/2-inch seam. Clip the seam salvage around the curve at a 1/2-inch spacing so the seam lies flat.
Turn the raw edge around the bottom and back of the cover ¼ inch to the wrong side of the fabric and pin to hold the fabric. Turn the edge another 1/4 inch to the wrong side and pin it to hold it in place. Sew the hem with a straight stitch to secure it.
Repeat each step to make an armrest cover for the other arm of the couch.