Measure the length of the chair from the center front at the floor, across the seat cushion and over the back to center floor behind the chair. Add 6 inches to the measurement for seams and tucking.
Measure the seat cushion width from the one side at the arm and across the cushion to the opposite side. Add 2 inches to the measurement for seams.
Measure the chair back width from the front-most point on the arm to the rear edge of the chair. Add 2 inches to the measurement for the seams.
Measure the arm height from the floor at the side of the chair and over the arm to the start of the cushion. Add 12 inches to the measurement for the seams and tucking.
Draw a picture of the chair with the measurements. Draft a picture of the three slipcover pieces. There is one large piece to fit the body of the chair from the front floor to the back floor and two arm pieces that attach at the side of the body piece.
Lay the body piece of fabric over the chair with the right side facing up. Attach the two arm pieces with pins, in the correct location, so the right sides are together. Sew the arms to the body with a 1/2-inch seam. Finish the inside seam edge with a serger or zig-zag stitch.
Lay the fabric back on the chair so the right side faces down. Pin the front and back edges of the slipcover together. Fit the slipcover over the curve of the wingback portion of the chair by marking the sewing width with pins or a pencil line.
Sew the pinned seams, along the markings, with a straight-stitch. Finish the seam edges with a serger or zig-zag stitch.
Fit the slipcover over the chair, turn the bottom edge under 1/4 inch and secure with pins. Turn the hem under another 1/4 to 1/2 inch so the bottom edge falls at floor level. Secure the hem with pins and sew close to the inside edge with a straight-stitch.