I have put zippers in Totes and bags a lot before now, you need a zip slightly shorter then the bag, press the top and lining of the bag in 1/4 of an inch, I trim the wadding back slightly so its not too bulky. Insert the zip between the top two layers and edge stitch. I must admit Its one thing I do baste to make sure it stays in place. Open the zip and then carry on in the same way as jenny did. Never failed yet. Its a bit fiddly but works.