For the bag:
Cut four panels of co-ordinating fabric about 6 inches by 5 inches and sew together in pairs on the shortest sides. Turn one inside out and put it inside the other, matching up the seam lines.
For the tabs:
Cut two strips about 4inches by 2 inches and turn in three sides about 1cm. Glue the loop half of the velcro to the strips loop side up and leave to dry. Cut two small pieces of the hook half of the velcro about 1 1/2 inches long.
When tabs are dry sandwich between the bag panels and stitch along one edge of the bag all the way around.
Turn it all the right way out and the tabs should be sticking upward. Stitch along the top 1cm away from the edge to secure the tabs and neaten the edge.
Turn inside out so velcro is facing outwards and stitch the bag panels together at the bottom. (if available - overlock the edge). Flatten the corners and sew them down leaving a triangle piece free.
Turn back the right way and lay it on the back of the book. Mark where the tabs sit and sit the hook pieces of velcro to the book.