Bring right sides of pieces together. Hold pieces in your hand with the 2 edges facing you and piece A closest to you. Work from right to left.
Step 1: Secure seaming yarn on wrong side of piece A at start of the seam. Pass needle through both pieces, from front to back in first stitch. Then bring needle back to front through the first stitch to the left.
Step 2: Pass needle to back, at the same spot worked in Step 1. Then bring needle to front in 2nd stitch to the left.
Step 3: Pass needle to back in first stitch to the right where the needle came through in previous stitch. Bring needle to front in 2nd stitch to the left.
Repeat Step 3 to complete seam. Secure end of seaming yarn.