How do I file taxes for my father, who died in 2018? Do I need to create a separate login, and just go through the process as if I were him?

[DDollar]

Yes.  You need to create a separate TurboTax account for him, if you are going to use TurboTax Online.  Early in the Personal Info section there is an option to indicate that the taxpayer is deceased.  Be sure to check that box.  Then just go through the process just as you would your own return.