First cut out your pattern pieces. I will be demonstrating with two rectangles and two rectangles with curves. I wanted to show you the difference between a straight seam and a curved seam.
From the front of two pieces sew the seam allowance towards the inside piece 1/8″ away from the seam. From the outside it looks like edgestitching next to the seam. From the inside it looks like two rows of stitches.
Press the seam allowance open the best you can. CLIP the curved seam allowance. Then press the seam allowance towards the facing or inside piece.
From the front of two pieces sew the seam allowance towards the inside piece 1/8″ away from the seam. You’ll need to adjust as you go to sew the curve. From the outside it looks like edgestitching next to the seam. From the inside it looks like two rows of stitches.