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?

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