Mattress Stitch is one of the most common finishing techniques for sewing seams. If done correctly, the finished look of this technique literally makes the stitches “melt” together.
To begin: thread a tapestry needle with yarn that matches the knitted fabric. Hold the two pieces to be sewn parallel to each other with the right sides of the fabric facing you and cast-on edge at the bottom. Loosely pin the pieces together to hold the edges in place.
Insert the tapestry needle from back to front between the first two stitches, just above the cast-on stitch; pull the yarn through, leaving a 3-inch tail.
Step 1: Insert the needle from front to back between the first stitch and second stitches of the second side and pull the yarn through.
Step 2: Return to the first side and insert the needle from the right side to wrong side where the yarn previously came out of the piece. Slip the needle upward under the next horizontal bar and bring the needle through to the right side.
Step 3: Cross to the other side and repeat the same process, going down where you came out, under one bar and up.
Repeat steps 1-3 until seam is complete, pulling closed every inch or so. Weave in the tail.