If you qualify to make a QCD, when entering the Form 1099-R that reports the distribution TurboTax will ask if you had any of the distribution transferred to charity. TurboTax will exclude from From 1040 line 4b with the "QCD" notation the amount that you indicate.
To be eligible to make a QCD you are required to have been age 70½ or over at the time of the distribution from the traditional IRA. Make sure that you have marked the IRA/SEP/SIMPLE box in agreement with the marking on the Form 1099-R provided by the payer.
As dmertz states assuming you qualify, after entering/editing the 1099-R form information and the IRA box is checked, you will arrive at a page titled "Do any of these situations apply to you?" Check the box indicating "I transferred all or part of this distribution to charity". This will open the proper interview for you to enter the charitable amount.