Before you start, you need to install a recent version of:
cvs rpm-build-macros rpm-build-tools
The latter contains the builder script itself.
To initialize your working directory, invoke:
builder --init-rpm-dir
builder will create all the necessary directories under ~/rpm/ and create initial config for cvs. This is essentially all the work needed to build the packages for your own use.
In case you also need to commit your changes back, edit the ~/rpm/packages/CVS/Root to contain your login instead of the anonymous cvs@ account.