Version 1: Sew a zig-zag stitch through the middle of each seam allowance. Then, trim off the raw edge of each seam allowance as close to the zig-zag stitches as possible. Press the seam open.
Version 2: Instead of sewing each side of the seam allowance, sew both sides together using a zig-zag stitch. Press the seam to one side, and trim off the raw edge as close to zig-zag stitches as possible.
Note: For sewing a single layer of fabric with a zig-zag stitch, it may help to loosen the machine tension a bit so that the fabric isn't pulled tight and puckered by the zig-zags.