Cut a strip of fabric 1 3/4 inches wide and at least 15 inches long if not more. You can trim it later to be appropriate for your specific bag.
Press the strip in half. Press again folding the edges toward the center fold.
Trim the ends. Run the strip thru the key holder. I've been known to run it through the wrong loop and make the entire strip. Don't do that dumb thing I did. You will only do it once. Don't try to sew the strip together the short way. It will NEVER line up. Instead, drop your stitch length to 1.5 and sew for a half inch on the length right down the center of the strip.
Give the cut "arrow" some fray bloc. Thread a full strand of embroidery floss about 30 inches long into a heavy, large eyed needle. I use a tapestry needle. Knot the end and trim. Put your needle between the two layers and pull to hide the knot between.
Insert the needle going back the other way, between the two straps to secure. Cut the floss where you come out. Wet the whole wrap with Fray Bloc.
You are done! Now just pin the strap in between the lining side seams up near the top and you are good to go! You want your strap to sort of have the key holder sitting on the bottom of the bag. If its too long it will knot and mess up. Now, when you use your new bag, just reach in on the side seam, grab the strap and pull out for your keys! ....Bunny