You don't. You enter the SSA-1099 into Turbo Tax and it automatically calculates the taxable amount depending on your other income.
Try going to the search box by My Account and type in Social Security Benefits. That will give you a Jump To link to take you directly to it.
OR Enter a SSA-1099, SSA-1099-SM or RRB-1099 under
Federal Taxes tab
Wages and Income tab
Then scroll down to Retirement Plans and Social Security
Social Security (SSA-1099. RRB-1099) - click the Start or Update button
