It's really easy - I simply sew the strips together using a 1/4" seam, adding enough strips so it is more than the length (or the width) of my cut purse piece.
Sometimes I'll cut the strips a little bigger first. For example, if the piece needs to be 13" (height) x 16" (width), take pre-cut jellyroll strips and cut them 14". Sew together seven strips to make a patchwork piece about 14" x 17.5". Trim to 13" x 16". Repeat to make 3 more pieces and you have enough to make a Reversible Bookbag! Iron interfacing on to the seamed piece.
Buy yardage that coordinates for the band, straps, and pockets. Often I will just buy solid black for this purpose. You can also buy yardage for the lining (reverse) side