You're having trouble with:
Cut yarn, leaving a long tail. Thread tail through remaining sts, pull together tightly and fasten off.
Sew back seam. Weave in ends.
Do that exactly. Cut the yarn, leave a coupla feet or so. Then take the loose end through each of the stitches still on your needle. Doesn't matter how you do it, you can use a crochet hook if you have one, or stuff it through with your fingers. When they're all threaded, pull up like a drawstring, then use mattress stitch as shown in the Finishing link I gave you above, to sew up the back seam.