Different approaches. You could check out the docs at the rpm.org website and read "Maximum RPM". IBM Devworks also has some good quick tutorials and then there's distro-centric (say Fedora), third party (say DAG) and generic RPM mailing list (archives). Or you could take any .src.rpm (best not take something ambitious like the kernel) and try to read the containing .spec file and figure out what it does. Unless you are in a hurry I'd suggest reading. Doesn't matter really, because if you're in a hurry I'd suggest reading too. It'll save time and aggrevation.
If you want to start right away and learn on the fly that's possible as well. State what you want to rpmify and we'll get you started. (Still that's no substitute for reading docs).