Set up: Anchor yarn on WS of piece 1 (or use yarn tail). With RS of both pieces facing up bring the threaded tapestry needle up into the edge stitch in the cast on or bind off edge (piece 1). Line up piece 2 and take the needle across to that piece, bringing the needle to the RS from the WS in the corresponding place on piece 2. Take the tapestry needle back to piece 1, coming up from underneath to form a figure-8. Rep between pieces until the bottom edge is closed.
Seam: Bring needle to RS of piece 1 going in at starting point, *run the needle under 2 horizontal bars of yarn and come up to the RS of the same piece. now go back to the last place yarn came out on piece 2, going into piece from the RS, and coming up to RS in a stitch; repeat from *. Pull yarn to close up seam as you go, also pulling it vertically to maintain stretch.