Once you do that, focus on one grouping at a time, writing lots and lots and LOTS of tests. Test everything, anything ... if it moves, test it. Heck, test it even if it doesn't move. (You want to make sure it doesn't start moving!)

While I don't think this is what you're proposing - I think this could be read as "write tests for everything in the legacy code before you change anything" which, IMHO, is a bad practice. As I said here

A counter productive practice that I've seen is to go through a large piece of legacy code and add developer tests for everything. Doing this with legacy code not driven by tests will produce a test suite that is brittle in the face of change. When you get to the refactoring you're going to find that you're going to be continually throwing away a lot of the new tests so you don't get any benefit from them.

In my experience it's much more effective to build the test suite around the changes you make to the code. Add tests when you add new features. Add tests around code that you're refactoring. Add tests to demonstrate bugs. In my experience just following those three rules naturallys build a test suite around the most important code.