Short version: you find very small things that you can write tests around (pulling messy logic out into testable functions, writing tested utilities that replace dirty hacks, etc.) and start building your testing framework starting with that.

Long version: Working Effectively with Legacy Code Book by Michael C. Feathers

It’s almost always bad as a new guy to come in and start refactoring.

There are lots of weird if checks to deal with a vendor that returns bad data over the api every Sunday night. Your new clean code is going to crash and burn in all of these cases.

That is not an argument against unit testing. That is an argument for applying it from the beginning…

Then refactor 50% of it.