Make RPMs from .spec and package source tarball? how?

[Ben Lancaster]

Hey,

How does one make an RPM from a source tarball and an RPM spec file?  
I need to patch the source of MythTV to fix a rather annoying bug,  
and then make an RPM based on the ATRPMS spec for the package. Of  
course, I could just go ahead and install from source, but this will  
mess up my yum upgrades/dependencies.

Thanks!

Ben