1) check out source from sourceforge SVN (done)

Patches should probably be made against: https://svn.boost.org/svn/boost/. I'm not how SF matches against this repo.

2) make changes
3) run tests??? (what is the best way to ensure the behavior has not changed?)

Add specific test files to libs/graph/test. Rebuild/run all of the tests - just type bjam. The testing framework will report any failures.

4) generate the patch (use svn diff?)

Yup.

5) submit somewhere

You can submit on the list. I'll review it, we'll discuss it and it will be accepted (or not). If the patch fundamentally alters the semantics of the concepts or default behavior of existing data structures, I probably won't accept it since it will break existing code.