I am trying to use the Turbo Tax website to open my 2018 return and calculate quarterly tax payments, but I can't figure out how to get into it. Can anyone help me?

[DoninGA]

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.  See screenshot

This will take you back to the 2018 online tax return.

Then use the procedure in this TurboTax support FAQ for creating 2019 Estimated Tax Forms 1040-ES - https://ttlc.intuit.com/questions/2584228-can-turbotax-calculate-next-year-s-federal-estimated-taxes