Some patterns come with instructions for a large shoulder strap that is not adjustable or permanently attached to the bag, such as the Logan Sling Pack. If you look at some of the tester photos you will see that the strap is permanently attached to the bag, but there is one tester that has trigger hooks on the end of the strap to make it removable.
To make one of those, lets start at the very beginning of an adjustable strap:
1. Collect the hardware you require - two trigger or snap hooks and one strap adjuster or slider. These are all to be the same "size" which means 3/4" hardware, 1" hardware, 1-1/2" hardware - you get the idea. The measurement on the hardware is the inside measurement where the strap will go. This tells you what size of strap you need to make - 1", 3/4", 1-1/2", etc.
2. Prepare the fabric / other materials you are making the strap out of. You want a piece of fabric that is 4 times the width of your finished strap - for a 1" strap, you need a 4" wide fabric strip; for a 1-1/2" strap you need a 6" wide fabric strip. Length of your fabric will depend on the body that bag will go on. For the John Duffel Bags I use a length of material that is 55". So for a crossbody strap that is 1-1/2" wide, you will want a piece of fabric that is 6" wide and 55" long. If using cotton, you will want this full piece of fabric to be interfaced with a woven fusible interfacing. If using a heavier material, you will want a strip of interfacing down the center of the material at least, it prevents the strap from stretching.
3. Once you have the cut fabric, press the end of the strap into the wrong side by 1/2”. Repeat for the other end of the strap. Then press it in half lengthwise. Open up the last fold and press the two sides into the center crease. Re-fold along the center crease encasing the raw edges in the center. You should now have a piece that is the finished width of your strap. Topstitch along all four sides. 
4. Now comes the hardware - take your strap and fold one end over by 1-1/2”, then slip the middle bar of the strap slider inside the fold. Topstitch across the end. Slip the opposite end of the strap through one of the trigger hooks, then bring it up to go under the bottom bar of the strap slider, over the center and under the top bar. Fold that open end over the other trigger hook by 1-1/2” and sew across the end.
You can now hook your finished strap onto your bag!