Remove any loose cushions on the sofa. Drape muslin fabric from the seat base over the seat back to the floor. Allow 2 inches to overhang the floor and the seat base. Drape a second piece of muslin across the seat base from the seat back to the front edge and down to the floor.
Pin the muslin along the joint between the seat back and seat base. Overhang the fabric 2 inches onto the floor. Drape fabric over the arm from the seat base to the floor. Pin the fabric to the seat base and along the seat back. Overhang the fabric 2 inches onto the floor. Repeat for the second arm.
Cut muslin to the shape of the front of the arm. Pin the arm front fabric to the arm and seat base sections. Repeat for the second arm. Check seams for straightness and fit. Remove the cover and sew your seams. Turn the cover right side out and fit it onto the sofa. If the sofa cover is loose, turn it inside out and correct the fit with more pins. Sew new seams and check the fit.
Mark the correct seams with a pen. Label each piece of the cover. Seam rip the cover. Pin your muslin pattern to your washed and dried decorator fabric. Cut out your pieces. Sew your cover together, face sides together following your correct seams. Turn the cover right side out and fit it on the sofa. Pin hems along the bottom edge. Sew the hems using a hem stitch.
Unzip the cover from the cushion. Seam rip the seams. Label each piece of the existing cover with tape and a pen. Iron the existing cover. Pin the cover to the fabric. Cut out the new cover pieces.
Sew the zipper into the back panel. Sew the two sections of the back panel face sides together using a basting stitch. Place the panel face side down. Pin the zipper centered on the basted seam, face side down. Sew the zipper to the panel using a zipper foot. Sew down one side, across the bottom three times, up the second side and across the top three times. Seam rip the basted stitch over the zipper to expose the zipper.
Sew the four side panels together along the short corner seams. Place the back and one side face sides together and sew a 1/2-inch seam. Continue with the other pieces until you have a circle of fabric. Place the top cover face side up. Align the side pieces to the top cover face side down so the corners match. Pin the covers together and sew a 1/2-inch seam all around the perimeter. Repeat for the bottom cover. Turn the cover right side out through the open zipper. Repeat for each cushion. Insert your cushions.