You don't enter the Form 3291 into TurboTax until you sell the stock. If you sell the stock less than one year from the grant date and two years for the date you acquired it, you will need to report ordinary income equal to the discount you were allowed when you acquired the stock. The Form 3291 will give you the information you need to factor that into the gain you recognize upon sale of the stock. You will input it in TurboTax when you report the stock sale that is reported to you on Form 1099-B.
If you hold the stock for a longer period of time, then all the gain will be capital gains so you won't have an issue with the discount so you won't need the Form 3291, as your capital gain is simply what you sold the stock for less what you paid for it. Also, the ordinary income from the discount upon sale of the stock, if any, will be reported on your W-2 form, so all you need to work on will be reporting the correct cost basis entered on your Form 1099-B, which normally requires adjustment.