How do I report a sale of private company stock that issued a 1099-B for the full proceeds, but will make installment payments over 5 years? Do I ignore the 1099-B and just report installment sale via Form 6252?

You would have to report the sale on form 6252 for installment sales. If the stock had been reported on an exchange as opposed to being private stock, you would not be able to use the installment method to report the income from the sale. You should also enter the Form 1099-B in the investment income section of TurboTax but list the cost basis equal to the sale amount. This will prevent the IRS from adding that income to your return when they match up the 1099 form income reported to them with your investment sales on schedule D of your tax return.

You can use the search icon on your menu bar in TurboTax and enter "Installment Sales" and you will see a "jump to" icon to find the installment sales income entry screen in TurboTax.