Pull all four leftover strands out through the FIRST joining stitch. This includes the two slipknot tails, and the two strands used for the seam. Braid them into a four strand plait (better explained elsewhere). The plait is for closing and not for looks so braid really tight. This will back the braiding up into the ball. The closer it gets to the last joining stitch the tighter your closing will be. Then tie it off with two small over hands, again tie these tight, melt the tips, and then stuff it all inside as far as it will go.