Measure from the floor, up the back of the chair, down to the cushion, across the cushion and down to the floor again. This is measurement A.
Measure from the floor by side of the chair up to the arm, down to the cushion, across the cushion, up the other arm and down the floor. This is measurement B.
Cut a large rectangle of fabric using measurements A and B as the sides. If necessary, cut two smaller rectangles and piece them together.
Hem the fabric by turning down the edges toward the wrong side twice, creating a fold ¼ inch deep both times. Sew the fold flat.
Throw the fabric over the chair, right side up.
Tuck the fabric into the folds of the chair and between the chair and the cushion if a cushion exists.
Gather the loose fabric up at the back of the chair and pin it so that the throw holds the chair's shape.