There isn't much to it, actually. Install it. If you're reading this then you've gotten the source. A simple 'python setup.py install' or 'python setup.py develop' will get you started.
Most of the time, you'll want to invoke intrumental something like this:
instrumental -r -s -a -t somename somepythonscript
That says, "Please run the script 'somepythonscript', instrumenting any module who's full dotted name starts with 'somename'. When you're done, please show me a report containing coverage percentages as well as a summary of all of the constructs you instrumented and the coverage results for those constructs".
More documentation is available at Read the Docs <http://readthedocs.org/docs/instrumental>.