Whipstitch seam

 

Place pieces with right sides together. Hold pieces with the two edges facing you.

Step 1: Secure seaming yarn on wrong side of one piece. Pass needle through pieces from back to front at start of seam. This creates a small stitch to begin seam.

Step 2: A little farther left, pass needle through pieces, again from back to front, wrapping seam edge. 

Repeat Step 2 to complete seam. Secure end of seaming yarn.