How do we pay the RMD tax on our IRA distribution that was not taxed when it was withdrawn?

The distribution is included on your tax return as ordinary income and is taxed at your current tax rate.

If your total tax liability is less than your total tax payments then you will receive a tax refund for the overpayment.

If your total tax liability is greater than your total tax payments then will have taxes owed.

If you have correctly entered your form 1099-R for the RMD, the tax on the RMD has been calculated by TurboTax and added to your tax liability.

If you have a refund, that refund is already after counting the tax on the RMD. 

If if have a tax due on your tax return, the amount due already includes the tax on the RMD. You do not tax on the RMD separately.