Set up: Anchor yarn on WS (or use yarn tail). Fold CO edge so that ends are toward you and CO edges are together. With RS facing up bring the threaded tapestry needle up into the edge stitch above the CO edge (on left side). Take the needle across to the right side, *coming to the RS from the WS in the corresponding place, then return to left side, repeat from *.
Seam: From the RS of the right side, *run the needle under 2 horizontal bars, coming up to the RS of the same piece between stitches. Now go back to the last place yarn came out on the left side, going into piece from the RS, going underneath 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.