Locate the last stitch at the end of a column of stitches. Insert your needle under both "legs" of the stitch, as seen in the photo at right.
Note: In the photos here, I have used a contrasting thread so you can see what I am doing. For actual seaming, use the same yarn as the pieces are knit with, or a matching embroidery thread.
Lay the second knitted piece parallel to the first. Locate the corresponding column of stitches on the second piece of knitting; insert your needle under both "legs" of the stitch at the end of that column.
Note: Your tapestry needle will always be pointing in the same direction as you are stitching: towards the end of the seam.
Pull the yarn through and pick up the legs of the next end-stitch of the next column on the first knitted piece. Be sure to work along the same row of stitches on each piece–don't jump from one row to another.
Pull yarn through and insert needle under next end-stitch of the next column on the second knitted piece. I've left my bright red yarn slightly loose so you can see what the stitches look like.
Continue in this manner, alternating sides, staying in the same column of bars on each piece.
Again, I've left the yarn slightly loose here so you can see what it looks like; you would pull the stitches firmly as in the next step.
When pulling the yarn through, pull it a bit firmly–enough to bring the columns of stitches together, but not so firmly that the fabric puckers. On the first few stitches, it may help to pull on both ends of the seaming yarn, as I am doing here.
Adjust the tension by gently tugging on the seaming yarn, and then gently tugging on the fabric to get the beginning of the seam to be flat. This is particularly important for the first few stitches of the seam to get the seam to be just right.