One of the wonderful things about sewing is that there is often more than one way to do things. I had this realization when I was making Betz White's Weekender Duffel. When she came to making the adjustable strap, she demonstrated a way to do it that was just much easier than the way that I've been doing it! While I was bummed to know that I've been teaching and sharing a technique that might be unnecessarily complicated, I'm very happy to have learned something new. I will share it here, and then go about the business of updating my sewing patterns and tutorials accordingly.
You will need:
1 1/2 yards light or medium weight cotton webbing
2 Swivel Hookss (D-Rings, sliders, or O-rings can also be used, but you will need a way to attach them to the bag, such as strap tabs)
1 Double-loop slider
Note: Make sure the hardware and webbing are all the same width
Thread one end of the webbing through the slider.
Pull about 2" of webbing through the slider. Fold the end in by about 1/4", then fold the webbing over on itself, concealing the raw edge. Sew in place with a reinforced line of stitching, a rectangle, or a rectangle with a "X" through the middle. You may find a zipper foot helpful to get as close to the slider as possible.
With the "right side" of the slider end facing up (as shown), thread one swivel hook through the other end of the webbing.
Bring the same end of the webbing around and thread it through the slider as shown.
Thread this end of the webbing through the second swivel hook, making sure that the "hook" part ends up on the same side of the strap as the slider as shown:
Pull about 2" of webbing through the swivel hook. Fold about 1/4" of the raw edge in, then fold the webbing over on itself. Sew the webbing in place with a reinforced line of stitches, a rectangle, or a rectangle with an "X" through the middle. A zipper foot may be helpful to get close to the swivel hook.
That's all there is to it! You are now ready to clip your strap onto your bag!
Update! After posting this tutorial I decided to update the free tutorial available for download in my pattern shop. I've replaced the original tutorial with updated instructions and photos instead of drawings. Drawing adjustable webbing straps is just too much of a headache for someone that is not a technical illustrator!