Lay your tea towel flat. Then, use a pencil (or whatever you plan to organize in your rollup) to measure where the fold would need to be (up from the bottom section). Pin on each side and sew up the sides. Backstitch at the beginning and end of each sewn line.

Continue to use your pencil (or other supplies you'll be storing) to roughly measure and decide how wide each section should be. Pin those sections and then sew each dividing line, running from the bottom of the rollup to the top of the fold in step one. Backstitch at the beginning and end of each sewn line.

Fold over your top section and decide how far you would like the fold to come. If your towel is just the right length you can skip this step. But if not, cut off excess fabric and hem to create a finished edge.

You can stop there or add a few more finishing touches. To make your flap a bit stiffer / easier to fold over, iron your fabric at the fold and sew a seam from side to side to make a flap that will lay flat.

(Optional) To keep the rollup secure, you can add a leather strip, tied in a knot. I didn't attach mine to the actual rollup - just wrapped it around and knotted it after everything was rolled. But you can sew the two pieces together, if it makes more sense to have that piece connected.