Measure the width of the widest part of the rocking chair back. Add 2-inches for side seams and 12-inches for a pleat. Measure the height of the chair back from the top to the floor. Cut muslin to this size.
Place the muslin flat on the work table. Measure the width to find the center. Mark the center. Measure 2-inches to left side of the center and mark. Fold the left side of the fabric to the right at your left side mark. Pin the fabric. Fold the same fabric back to the left at your center mark. Align the top edge and pin through all three layers.
Measure 2-inches to right side of the center and mark. Fold the right side of the fabric to the left at your right side mark. Pin the fabric. Fold the same fabric back to the right at your center mark. Align the top edge and pin through all three layers. You should have a pleat where the folds touch at the center mark. Place the fabric in the sewing machine and sew across the top of the pleat to secure it. Iron the pleat down the length of the fabric. Pin the pleats in place along the length.
Place muslin from the top of the chair back, along the face of the chair back and overlapping the chair seat 1-inch. Use your cut and sewn muslin with the center pleat to cover the chair seat back. Center the pleat facing toward the chair and pin the top edges of the front muslin to the back muslin and along the sides of the chair.
Drape muslin across the chair seat and down the front to the floor. Pin the seat muslin to the front and back muslin. Drape the arms last, following the same pinning technique. Label each piece with a marker. Remove the cover and sew the seams. Turn the cover right side out and fit it onto the chair. Mark a functional location for a button and loop to hold the pleat together on the real cover. The button will sew onto one pleat and the artful loop and stationary button will sew onto the other, but once buttoned, they should appear centered. Correct the fit as necessary.
Rip the seams on the muslin cover with a seamripper. Pin your muslin pieces to your laundered and ironed slipcover fabric and cut out your pieces. Sew your pleat together first. Iron and pin your pleat. Sew your cover pieces together. Ease your curves and clip your seam allowance corners to remove bulk. Turn the cover right side out.
Place the cover on the rocking chair and fit your hem. Pin and sew the hem using a hem stitch. Sew two buttons on either pleat along the bottom of the chair back. Space the buttons apart using the length of the figure-eight loop as a guide. Open or unbutton the loop to put the cover on. Loop the button to close the pleat while the cover is in place.