Three-needle bind-off technique:
A join that produces what looks like a perfect seam and is performed using three needles — two that are holding the live stitches of the pieces to be joined, and a third used for working the bind-off. The basic action is almost identical to that of a standard bind-off. This is a wonderful way to join two pieces of knitting in place of sewing a seam, especially effective when used to join shoulders. The method is also referred to as binding off two pieces together.

To perform this technique, place stitches that may be on hold onto two needles, so that right sides of the fabric face each other and the tips of both needles point in the same direction, If you are joining shoulders, make sure you carefully match left shoulder to left shoulder and right to right. Hold these two needles parallel to each other in your left hand to act as the left-hand needle. Using a third needle as the right-hand needle, insert this right-hand needle through the first stitch on the front needle and the first stitch on the back needle in your left hand. Knit the two stitches together as one and slide them both off their respective needles. Repeat, creating a second stitch on the right-hand needle. Then perform the first bind-off by slipping the first stitch worked over the second stitch worked and off the needle. Repeat the process until all stitches are bound off. Cut the yarn, leaving a 6"-10" (15-25 cm) tall and draw the tail through the final remaining loop. Weave in the end.

If you are having a hard time visualizing this process, give it a try. If you are still not getting it, please consult a well-illustrated knitting reference book or ask a knitting friend for a demo.