Write the Package Specification

Best to start with an existing one or a canned template or possibly a GUI rpm builder like krpmbuilder
The basic sections of the rpm specfile: Header, %description, %prep, %build, %install, %pre, %post, %preun, %postun, %files, and %changelog
Technically, only the Header and %description are required, but you will get no binary rpm without a %files section (which can be empty).
List all Sources and Patches appropriately.  Patches need to be specified as such in order to be referenced by number later.
Write pre and post scripts for install and uninstall as necessary.  Try to keep this to a minimum length -- rpm is powerful enough to to meet most needs usually met by these scripts.