Lay your long strap piece on the bag starting from the center of the fabric.
So, I made this duffel bag for him.
Now, all we need to do is stitch the circle pieces to each end.
Iron down Insert the interfacing in the center of the straps.
Grab your large outer piece and your strap.
Sandwich them between the other side of the zipper-the part that is not sewn yet-with the zipper facing down on my main fabric.