Well, I haven't done much hand piecing, so take my answer with a grain of salt
I *think* that when hand piecing, your template is the size of the finished piece. you mark around the edges with a pencil and then cut out the piece, adding the seam allowance by eye. You would mark the center point of each curve. Wouldn't you match the marked seam lines and pin the pieces together, like you would a sleeve in a garment? Then sew along the marked line on whichever side you want to be toward you.
I know that Jinny Beyer has a book on hand piecing, she does not mark her seamlines- however, she has vast experience at this and probably could cut a 1/4 inch seam allowance with her eyes closed!
Are there hand piecing tutorials out there on You Tube?
Kathy