This tutorial is for a case that will fit an iPhone WITH a protective case on it. If you don't have a case, you will want to take 1/4" off the overall width dimensions for the exterior and interior pieces of fabric. You can leave the height as is.
The case has a flap to keep the phone inside (although you don't HAVE to add it), and it has a tiny strap with a clasp so you can hook it wherever you want. Of course, you can make this strap longer so it becomes a wristlet....go ahead and modify it to your liking. You can even add a small pocket for a credit card or whatever.
While my tutorial does not give instructions on adding the pocket, I do mention at what point you should add it if desired.