IMO you'll need to have tests setup, either you develop unit tests if possible, or you gather manual test case scenarios which you can replicate over and over.

IMO you'll need to have tests setup, either you develop unit tests if possible, or you gather manual test case scenarios which you can replicate over and over. Then start refactoring, usually by the last (20-30% ish?) part of code, moving above. IMO, it's easier to take out / refactor last part since the variables declared inside that part isn't used anywhere else .