Hello Watson ... Take a look through posts from MaryKatherine (from Guelph). She did one a few years back and then later added to it to get to the new size she wanted.
A thought ... Maybe quilt the add-on fabrics first. That way you know for sure how much shrinkage you will have from the quilting. A clean cut down the sides of the main quilt and the add-on, will let you lay them flush, side by side and then machine stitch on with a matching or even a contrasting fabric front and back, as wide or as narrow as you want. For each add-on you could do three rows by machine and no stitching showing. The 4th would either be stop stitched, or you could hand stitch, as you would for a binding edge. Then repeat for the other side, and then the top and bottom.