If you're working with a big ball of mud, I find that the best approach is to immediately start doing TDD with hermetic end to end tests.

Hermetic = could run just fine on their own if run on a freshly installed OS that is cut off from the internet.

The first tests you build this way will be extraordinarily expensive (faking databases & http calls is fiddly), but they pay enormous dividends.

Once you have a large enough body of these and youve refactored some clean interfaces underneath, you can start writing future tests against those.