TurboTax says "You can claim 50% of what you pay in self-employment tax as an income tax deduction" using form 1040. How exactly do I do this using TurboTax?

[DoninGA]

It is not a deduction but an Adjustment to Income that reduces your Adjusted Gross Income.  This is done automatically by the TurboTax program if you have Self-Employment taxes on your tax return.

The adjustment is entered on Schedule 1 Line 14 and flows to Form 1040 Line 8a

[VolvoGirl]

Turbo Tax automatically does that for you.  

UPDATED WITH 2019 LINE NUMBERS....

Self Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment.  You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400.  The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare.  So you get social security credit for it when you retire.  

The SE tax is already included in your tax due or reduced your refund.  It is on the 1040 Schedule 2 line 4 which goes to 1040 line 15.  The SE tax is in addition to your regular income tax on the net profit.  You do get to take off the 50% ER portion of the SE tax as an adjustment on 1040 Schedule 1 line 14 which flows to 1040 line 8a.  Turbo Tax automatically calculates the SE Tax and Adjustment.

For 2019 Schedule C Net Profit or Loss now goes to 1040 Schedule 1 line 3.  Then the total on schedule 1 line 9 goes to 1040 line 7a.

[Critter]

Updated for 2019 :

You do get to take off the 50% ER portion of the SE tax as an adjustment on 1040 Schedule 1 line 14.   The SE tax is already included in your tax due or reduced your refund.  It is on the 1040 Schedule 2 line 4 which goes to 1040 line 15.  The SE tax is in addition to your regular income tax on the net profit.