Roth conversions are not "related" to any tax year other than the year in which the conversion occurs. The Form 1099-R reports the amount total converted in 2018 and that's what needs to be reported as converted. Simply enter the Form 1099-R under Wages & Income -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plan Withdrawal (1099-R) and answer the follow-up questions to indicate that the money was moved to another retirement account and the amount converted. Later, click the Continue button on the Your 1099-R Entries and answer any questions regarding your basis in nondeductible traditional IRA contributions and your year-end balance in traditional IRAs.
If you had basis in nondeductible traditional IRA contributions for some years before 2018, the amount of basis from your most recently filed Form 8606 carries forward to line 2 of your 2018 Form 8606 to be used to calculate the taxable amount of your 2018 Roth conversion. For example, if you made a nondeductible traditional IRA contribution for 2017, the amount on line 14 of your 2017 Form 8606 is the basis that carries forward to line 2 of your 2018 Form 8606.
