MATTRESS STITCH

Thread your yarn on to a needle, then weave along an edge to where you want to start. Bring it to the front between the first and second stitches of this edge, then insert the needle between the first two stitches of the other edge. Bring it back up through the opposite piece, between the two stitches and below the horizontal strand you just made. Keep stitching in this way, pulling the yarn tight every few stitches.