Also known as: seaming, grafting.
Step 1: Take a darning or tapestry needle pre threaded with yarn and insert it into the bottom corner of one of the pieces you are seaming. Insert going from the wrong side to the right side of the fabric.
Step 2: Pull the yarn through, leaving about six inches, bring the needle around to the wrong side again. Insert the needle into the same hole you first put the needle through and pull tight. This will lock the yarn in place.
Step 3: Take your needle and pick up the two ladders together between the first and second row, going vertical, on the opposite piece of fabric.
Step 4: Pull tight and take your needle and pick up the next two ladders together between the first and second row, going vertical, on the opposite piece of fabric.
Repeat steps 3-4 until you have seamed all the stitches together.
Example of the Mattress Stitch
music in video is from demo 2008 by (Latch\xe9 Swing) / CC BY-NC-SA 2.0