Where do I report a Roth backdoor conversion done in April 2019 using contributions assigned to 2018 tax year?
You report the traditional IRA contribution on your 2018 tax return. If it's a nondeductible contribution, 2018 TurboTax will prepare Form 8606 Part I to report the nondeductible contribution, otherwise it will appear as a deduction on Schedule 1 line 32.
You will report the Roth conversion on your 2019 tax return by entering into 2019 TurboTax the 2019 Form 1099-R that you will be receiving early in 2020. When you transfer in your 2018 tax file to begin your 2019 tax return, TurboTax will transfer in the basis that appeared on line 14 of your 2018 Form 8606 to be used in determining on 2019 Form 8606 Parts I and II the taxable amount of your Roth conversion.