When it's time to sew seams on your knitted project, knowing a few simple seaming techniques can make your finished garment look polished and professional. The mattress stitch is a very easy (and satisfying) join. Also know as an Invisible Seam, it joins the knitted pieces without showing a seam on the front of the work. Here's how it's done in just a few steps.
Start by making a firm join at the lower edge:
Step 1: Thread the needle with the yarn you are using to seam (if you leave yourself a long enough tail when casting-on you can use that strand).
Step 2: Place pieces to be seamed side by side - right sides up. Using a flat surface to support your pieces works best.
Step 3: Locate the stitch 1 in from the edge stitch (your edge stitch will curl toward the inside). The 1 stitch in is where your seaming will be done.
Step 4: Insert the needle from back to front into the corner stitch (1 stitch in) of the piece without the tail.
Step 5: Insert the needle from back to front into the stitch with the cast-on tail. Pull yarn gently to pull edges together.
Step 6: Now from back to front, insert the needle into the stitch on the piece without the tail (right back where you started). Again pull yarn gently to draw the edges together and lock the 1st stitch.
Step 7: Your yarn will now be on the right side. Locate your stitch 1 in from the edge (the stitch will look like a V) and locate the bar at the bottom of the V. Now insert your needle just as in the image above and draw the yarn through.
Step 8: Move to the corresponding stitch on the opposite piece and locate that bar. Draw your yarn through.
Step 9: Continue alternating sides until you reach the top of the piece, then fasten off.
As you progress through the seaming, occasionally stretch your piece to make sure your seaming is not too tight which will cause your project to pucker.
This is how your seaming will look from the front and the back.