If you want to make a simple and quick-to-make bag then why not download this pattern and make this very casual hipster / cross-body / shoulder bag. There are 2 sizes included in the pdf downloadable pattern with the bigger one being 1 ½ inches taller than the smaller one.
The bag is very much aimed at the casual you. It's relatively lightweight and is ideal for shopping, cycling or strolling in town. However, it does have lots of ample room for your phones, clutch, keys and even a small book or tablet. If you want to make the bag even easier and quicker you can miss out the recessed zipper part and add a magnetic snap, buttons, velcro, etc.
The approximate size of the finished bags are as follows: