Thread a needle with a 9-inch length of double-knotted thread. The stitch we use for this technique is similar to a blanket stitch, shown here; just make the stitches right next to one another rather than spacing them out.
With the outside of the garment facing you and the buttonhole positioned horizontally, pass the needle from the inside to the outside, just below the original stitching and about 1/8 inch below the slit. Pass the needle down through the slit and back up next to where you started, keeping the thread under the tip of the needle and holding it with your thumb as you pull the thread gently until it lies flat. Continue stitching until you reach the end of the buttonhole.
To stitch the ends, shift the garment so the buttonhole is vertical. Create a horizontal stitch (perpendicular to slit) by passing the needle from right to left 3 times. Then stitch vertically until horizontal stitches are completely covered.
Shift the garment so buttonhole is horizontal; repeat steps 1 and 2 to stitch the other side.