One of the most common methods of seaming is Mattress Stitch. This is excellent for most sweater seams and leaves a small but comfortable inside seam. It is sometimes called "Invisible Seam" because it almost disappears. Thread a yarn needle with matching yarn. With right sides facing you, place edges to be seamed together.
Beginning at hem edge secure yarn through both sides. (Steps 1 & 2) With needle pointing up, place point into last stitch and take a small stitch, leaving yarn loose (Steps 3 & 4); go to the opposite piece and pick up the corresponding row (Steps 5 & 6). Return to the first side, enter the lst point of exit and pick up a stitch (Steps 7 & 8). Be sure to keep needle pointing up to the top of the work rather than to the side of the work. Tighten yarn in seam after picking up 3 or 4 stitches. Yarn must be tight to disappear, but not enough to shorten seam. Continue stitching until seam is finished.
© Reprinted with permission from Craft Yarn Council of America's Certified Instructors Program