Decide how width the bag should be, for example, 5 inches wide. Now cut a rectangle with the width chosen, by a length equal to two times the width (10 inches long).
Calculate 15 percent of the length to get the length of the top flap. (0.15 × 10 inches = 1.5 inches)
Mark the top flap’s length with pins. Fold and press the flap with wrong sides together and remove the pins.
Open the top flap and fold the bottom edge under the top flap with wrong sides together. The bottom’s cut edge should be about 1/4 inch below the pressed top flap fold. Press the bottom fold.
Stitch each side edge through all layers. Back-tack at both ends of each side seam. Now use a rotary cutter embellishment blade (curves, zigzags, etc.) or pinking shears to create a decorative top flap edge.