Sew the exterior cloth pieces together (wrong side facing up) with a seam width of 1 cm.
Press the seam allowances towards the brown cloth and turn the cloths so that the right sides are facing up. Sew a single line of topstitching through the right side of the brown cloth and the seam allowances.

Fold both exterior and interior cloths inside out (with right side facing right side).
Sew a 5 mm seam along each of the sides. Open the seams and iron flat.
Sew a boxed corner with a 2 cm width perpendicular to the seam (as shown in the diagram) and cut away excess cloth.

Place the Interior cloth (wrong side round) into the Exterior cloth (right side round) so that wrong side touches wrong side when put in. Sew the tops together on each side using a blanket stitch.

Affix the clasp to complete.