Fear the mattress stitch no more! Here's a step-by-step tutorial to help you master this easy seaming stitch.
For seams worked vertically (such as a side seam): Locate the "bars" between each column of stitches. In the photo at right, I've caught one of these "bars" on my needle. You will be stitching using these bars all the way down the seam.
A tutorial for working horizontal seams in mattress stitch (such as a shoulder seam) is here.
Locate the first bar inbetween the very edge column of stitches and the next column.
Pick up that bar using your threaded tapestry needle. Note: 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.
Left-handed folk: I'm right-handed, so I am working the seam from right-to-left. Feel free to work from left-to-right if that is more comfortable for you.
Lay the second knitted piece next to the first. Locate the first bar next to the edge column in the second knitted piece, and pick up that bar with your needle.
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 next bar on the first knitted piece. Be sure to stay in the same column of bars–don't jump from one column to another.
Pull yarn through firmly. I've left my bright red yarn slightly loose so you can see what the stitches look like.
Find the next bar in the same column on the second piece and pick that up with your needle.
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 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.
Particularly for the first few stitches, adjust the tension by gently tugging on the seaming yarn, and then on the fabric to get the beginning of the seam to lay flat.
Continue stitching, gently pulling each stitch so that the columns meet at the seam line.
Here's what the seam looks like from the front. If it's done properly, the seam should be practically invisible.