Overcastting Seams

The overcast seam finish by machine is used on fabrics that fray.  It is a simple zigzag stitch going on and off the edge of your seam allowance, and is the most commonly used seam finish.

Sew a 5/8" seam allowance down one side where you pinned.

Press seam open. This is what your over-casting seam finish should look like.

Looking into more detail about the overcastting seam, the depth and width of your zigzag stitch should depend on the type of fabric you are working with.  On fabric that frays a lot you can turn the raw edge under before zigzagging, this helps prevent the fine fabric from getting caught and jamming in your sewing machine.