After entering the Form 1099-R exactly as received under Wages & Income (or Personal Income) -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plans (1099-R) and answering the immediate follow-up questions to indicate that you converted the money to Roth, click the Continue button on the Your 1099-R Entries page and, when asked, indicate that you made nondeductible contributions to your traditional IRAs. TurboTax will then ask you to enter the amount of prior-year nondeductible contributions from your most recently filed Form 8606 and will ask for your total 2017 year-end balance in your traditional IRAs. TurboTax will use these entries to prepare Form 8606.
