There are several patterns out there for making fitted diapers. Or you could also just buy a few diapers to get an idea of how they're made and create your own, which is fun if you (or your friend) are the creative type, and then you can customize the diapers to fit exactly how you want on your child. The first few diapers I made, I tried just from recycled household fabrics (old t=shirts, towels, whatever I could get my hands on that was essentially free) and the first few dipes I made were pretty funny looking, but fully functional. I tried a bunch of other WAHM diapers to get a feel for what I liked and didn't like, and I'm still experiementing with sewing and adjusting diapers and feel like I've gotten pretty good at it. It is fun to try new styles and new materials.
SO, to answer your question, you could go either way, buy a pattern or just create your own. My best bit of advice is to try some cheap materials first so you don't feel like you're "ruining" any fancy expensive fabrics and most of all, have fun!