First you'll need to obtain Cantiere sources. Latest sources are available via download page.
After install you should have directory structure like this:
Create a folder with name "specs":
In that folder put spec file you want to build.
If you have additional resources you want to use in your spec file (patches, source code packages) you can put it in src directory.
After that you can run rake -T command to check if you spec file is picked up:
[boxgrinder@domU-12-31-39-03-BE-17 cantiere]$ ls specs/
[boxgrinder@domU-12-31-39-03-BE-17 cantiere]$ rake -T
rake rpm:all # Build all RPMs
rake rpm:repodata:force # Force a rebuild of the repository data
rake rpm:sign:all # Sign all packages.
rake rpm:torquebox # Build torquebox RPM.
rake rpm:upload:all # Upload all packages.
Now you can run rake rpm command to build you RPM:
[boxgrinder@domU-12-31-39-03-BE-17 cantiere]$ rake rpm:torquebox
I, [2009-12-23T05:00:51.573650 #1860] INFO -- : Building package 'torquebox-1.0.0.Beta16-1.noarch.rpm'...
I, [2009-12-23T05:01:22.899354 #1860] INFO -- : Package 'torquebox-1.0.0.Beta16-1.noarch.rpm' was built successfully.