Measure the length of the chair body in inches. Start on the floor in front of the chair, across the seat, over the back and end at the floor behind the chair.
Measure the width of the chair body in inches. Start at one end of the seat cushion and measure across to the opposite side. Do not measure the chair arms.
Measure the length of the side of the chair in inches. Start at the floor on the chair side and measure over the arm to the start of the seat cushion. Use this measurement for both sides of the chair.
Measure the width of the side of the chair in inches. Start at the front of the arm and measure to the back of the chair.
Sketch a miniature pattern using the measurements. A home-sewn slipcover requires three pieces of fabric: one for the chair body, and two for the chair arms. This will give you a visual of each chair section.