Measure the chair back at its widest point; add 1 inch and call this measurement A. Measure the height of the back from the top to the seat; add 1 inch and call this measurement B.
Cut two pieces of fabric that measure A wide by B long.
Position the fabric pieces together with the right sides facing one another. Pin around the edges to hold them together using straight pins.
Use your sewing machine to sew a straight seam up one side, across the top and down the other side of your chair cover. Position your seam 1/2 inch in from the outside edge of the fabric, then go back over the edge with a zigzag stitch to make it extra sturdy.
Fold the bottom edge of the slipcover toward the wrong side of the fabric by 1/4 inch, then 1/4 inch again. Sew a straight stitch around the folded edge to create a hem.
Turn the cover right side out so it resembles a pillow case. Slide it down over the back of your chair to cover it.