The most important thing in developing software with TDD is always starting with tests. That's the only way you can ensure that code units are easily testable. This also naturally encourages good design practices like the single responsibility principle or the inversion of control. Sticking to those principles helps in writing good and maintainable code. And we've already seen how hard it is to test code reliably when tests are just an afterthought.
But caring about software correctness and maintainability does not end with testing and quality automation. These two allow us to verify the requirements we know about and fix bugs we have discovered. We can of course deepen the testing suite, and we've learned that mutation testing is an effective technique to discover potential testing blindspots, but this approach has its limits.
What follows next is usually the constant monitoring of the application and listening to bug reports submitted by the users...