Due to our income, we can only contribute to a non-deductible IRA. With the back door conversion, this would allow us to convert it to a Roth IRA. I only found instructions for TurboTax2016 but is not applicable to this year, 2018 version.
Nothing has changed between 2016 TurboTax and 2018 TurboTax with regard to this.
The contribution and the conversion are separate transactions:
The traditional IRA contribution for 2018 is entered in 2018 TurboTax under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions. Since it's nondeductible it creates basis in nondeductible traditional IRA contributions on Form 8606.
The Form 1099-R reporting the Roth conversion is entered under Personal Income -> Retirement Plans and Social Security -> IRA, 401(k), Pension Plans (1099-R) and you indicate in the follow-up the amount that was converted to Roth. If the Roth conversion was done in 2018, this will be done in 2018 TurboTax. If the Roth conversion was done in 2019, the basis from your 2018 Form 8660 will transfer to your 2019 Form 8606. In either case the Roth conversion will appear on Form 8606 Part II.
Under the 2018 tax law, taxpayers are no longer able to recharacterize any Roth IRA conversions done in 2018 and onward. please view the link below for more information.