Edgestitching can easily be confused with topstitching. Edgestitching is sewn 1/8" from the edge, while topstitching is 1/4" from the edge. Edgestitching is also a decorative stitch on the face of the fabric. To make edgestitching easy, use a #5 foot. It's the foot that has a little "fence." Before I start sewing, I do this to my bobbin.
Line the fence up with the seam and move your need to the left or right depending on where you want your stitching and where the seam allowance is. In my example, I have my needle to the right.
Sew along the seam, keeping the fence in the seam. It should look like this.