Make a template for the chair pocket. Use a piece of paper or scrap cloth to trace the outline of the chair back, from seat to top. Add a few centimeters all around for a seam allowance.
Cut fabric to the template's measurements. You will need twice as many pieces of fabric as there are chairs. If the fabric has frayed edges, stitch them to keep the fabric from unraveling.
Finish the bottom edge of each piece of fabric, since the bottoms will not be sewn together.
Cut rectangles of fabric to form the pockets. These can be as wide as the larger piece and and 1 inch shorter, or just large enough to fit papers and folders, about 10 inches by 13 inches. Finish the top edge of each rectangle, since it will not be sewn to the chair cover.
Stitch or glue the pockets to half of the larger pieces of fabric, leaving the tops of the smaller pieces unattached for the pockets' openings. For large pockets the same width as the chair cover, sew the pockets to finished bottom edge and to both sides. For smaller pockets, sew them in the center of the larger piece.
Align two pieces of large fabric, one with a pocket and one without, face to face with the pocket facing in toward the other piece. Sew the two pieces together on three sides, leaving the finished bottom edges unattached. You can also use fabric glue to attach the two pieces in lieu of sewing.
Turn the cover inside out and fit it over the back of the chair with the pocket on the outside.