You can always amend 2018 to include the 8606, just to be on the safe side. Below are the steps for the entry of a "Backdoor Roth" as per Forrest.
The process is the same on the Desktop version of TurboTax ... just stay in the Step-By-Step mode and don't use the Forms mode.
Doing a "backdoor Roth" in TurboTax is a 2-step process.
First, go to Federal Taxes > Deductions & Credits > I'll Choose What to Work On > Retirement and Investments > Traditional and Roth IRA Contributions. Enter the contribution to the Traditional IRA (TIRA) that you made. Depending on your income and if you have a retirement plan at work, TurboTax will either tell you that your contribution is not deductible or it will ask you "Do you want to make your IRA contributions nondeductible?"
Once TurboTax knows that your TIRA contributions are nondeductible (that is, you have a basis in your TIRA), then enter your 1099-R at Federal Taxes > Wages & Income > I'll Choose What to Work On > Retirement Plans and Social Security > IRA, 401(k), Pension Plan Withdrawals (1099-R). For a Roth conversion, the 1099-R usually has a Distribution Code 2 in Box 7. That means it is an early distribution but an exception applies so there is no penalty.
