You will need:
Old button-up shirt (not stretchy material)
Sewing machine, thread
Needle & Thread (if you want)
Step 1: Cut Out the Pieces
Then, cut out the strap from the button placket. Cut right along the edge (see picture) to the desired length.
Step 2: Sew!
Next, pin and sew the sides, outside in. Make sure you stay pretty close to the edge so you still have room in your bag!
Step 3: Trim & Turn Inside Out
The next step is to attach the strap. I didn't like how I did mine so I'm not including a picture. I think it would work best to sew it by hand using a whipstitch. I just sewed straight across which is functional, but doesn't look great.