There is no limit on the amount you are permitted convert (other than being limited to the amount in the traditional IRA).
VolvoGirl's comment above describes how to reach the page to enter a Form 1099-R to report a Roth conversion. On TurboTax's 1099-R form enter:
For maximum benefit of the conversion, you'll want to decline tax withholding on the conversion and pay the taxes with other funds. That means that you might need to pay estimated taxes to avoid a tax underpayment penalty. If you are under age 59½, have taxes withheld, and do not substitute other funds to complete the conversion of the amount withheld for taxes, the amount withheld for taxes will be reported on a separate code 1 Form 1099-R with the withholding amount in boxes 1, 2a and 4, subject to a 10% early-distribution penalty.
Remember that the Tax Cuts and Jobs Act of 2017 eliminated the possibility of recharacterizing a Roth conversion, so Roth conversions can no longer be undone.
For this to be 2018 income, the deadline for making the distribution from the traditional IRA to make the conversion is December 31, 2018.