I decided to watch more videos before I began sewing. I saw one which taught how to add the zipper after the bag is done. That sounds like a good idea to me. It's the method where you put fabric on the sides of the zipper and then attach that fabric to the bag.
That way I can make the bag they way I want, with the side mounted single shoulder strap and then add the zipper afterwards.
We'll see how that goes!!
Thanks for everyone's help so far.