Put the patch name in ~/rpmbuild/SPECS/kernel.spec just before END OF
PATCHES.

Run rpmbuild -bb kernel.spec

You will have the kernel rpm files in ~/rpmbuild/RPMS/x86_64

The gotchas are left as an exercise for the reader (if you are a
newbie, there will be lots of them).  :-)  And it's rough, there is a
lot of optimization that I left out.

So it's a lot of work, but a great learning experience if you are up
for it.