I've already filed my 2018 return via an accountant, but I'd like to start using TurboTax for 2019. How do I go about filing estimated taxes for 2019 via TurboTax?

Enter your 2018 account data into Turbo 2018 to start your profile.
You might be surprised to find Turbo could have found more deductions. When doing 2019 you'll import in your 2018. Once that's done its easy to to do your estimates.