Illustration of Invisible Sewn Seam

Place the 2 pieces right sides facing up (that's the side you want to show), next to each other, matching stitches across the side edges. 

Thread needle with a length of yarn. With the needle, weave the yarn though the stitches on one piece, bringing needle out at the corner to begin sewing. Leave a few inches of yarn woven through fabric to secure. Do not make a knot. 

Insert needle through the corner of opposite piece and draw yarn through. 

Insert needle through next row-end stitch on the first piece and draw yarn through. 

Continue to sew up the seam, working in a zig zag pattern for the length of the seam. Weave yarn through several stitches to anchor it, then cut tail.