Begin with a piece of fabric 6 inches by 3 3/8 inch that has the stitched design centered on it. Lay it on a surface face up with the bottom edge toward you.
Fold so the left edge lines up with the right edge. Sew a 3/8 inch seam across the top.
Turn the bookmark right side out so that the seam runs down the center back. Press the seam open and flat.
The bottom front edge and the bottom back edge should line up perfectly. If not then it they can be trimmed. Finish the bottom edge with a narrow hem or decorative trim. Or both.
Alternatively, the bottom edge could be blanket stitched and trimmed BEFORE the sides are sewn together.