When sewing seams, there are two steps. First, prevent fraying by finishing the edges with an overcast stitch. Then use a running stitch to sew the pieces together.
Overcast the outside edges of the seam allowance.
Press the overcast edges.
Using a C-Thru ruler and a Marks Be Gone pen or pencil, draw in the seam line.
Matching the edges of the seam allowance, right sides together, pin the pieces together.