Either is a good path. There's a
fowler pouch pattern here on the site, with pdf downloads for each piece. It's a dandy bag, but a little more complicated than a simple bag. Read and think it through before sewing. Check the craftsman section here too, especially back threads for more examples and links to patterns. Lots of example photos and good ideas. There are some great books out there too.
Two quick words of advice:
Numero uno, I made my first one from cut-up blue jeans, just to get a feel for the way things went together. Dirt easy and turned out a useful shoulder bag, PC or not, in a couple of hours.
Numero dos, figure on making several if the fever bites you, so don't be too hard on yourself if you get a ****oo here and there. Each one you make will be better than the last. I'd start with a cheap piece of "learning leather" to learn all possible mistakes, then go on to a good piece for the bag you really want.