How do you sew an invisible seam in knitting?

Start by placing the 2 pieces of knitting together so the edges line up. Pull out yarn from the skein you used to knit the pieces. Measure the length of the knit pieces you'll be stitching together and pull out 3 times that length. Cut the yarn and thread it through a blunt needle so that 4 inches (10 cm) are pulled through the eye to keep the yarn in place. Then, tie a knot at the bottom of the yarn tail so it's secure when you pull the yarn through the bottom of the left piece. Insert the needle through the bottom corner of the right piece and pull the yarn through. Pull the edge of the left piece to reveal a gap with horizontal bars, insert the needle below 2 of the horizontal bars near the bottom of the piece, and pull the yarn up and through. Pull the edge of the right piece so you can find the gap with the horizontal stitches. Insert the needle under the bottom 2 horizontal bars and pull the yarn through. Then, alternate the mattress stitch between the left and right pieces of fabric. Pull the yarn to bring the two pieces together and continue to mattress stitch in 2 inch (5 cm) increments until you're finished. Complete the job by tying off the yarn at the end of the seam.