How do you file taxes from 2018?

To complete and file a 2018 tax return using TurboTax you would need to purchase, download and install on a personal computer one of the 2018 desktop editions from this website - https://turbotax.intuit.com/personal-taxes/past-years-products/

A 2018 tax return can only be printed and mailed, it cannot be e-filed.

With TurboTax, you would need to use one of the previous years' products (which must be installed - there is no online version for past tax years).

See https://turbotax.intuit.com/personal-taxes/past-years-products/