How to enter a capital loss depend on if you are referring to a capital loss carryover from a prior year or a capital loss from a sale for this current year.
1. If you used TurboTax for your prior year, then capital loss carryovers should have transferred over. If you need to manually enter the carryovers, then you can do so following the directions below.
Easiest way to get to this section is to -
Alternatively to the search method, you can get to the same section by clicking the tabs Federal tax -> Wages and Income -> Investment Income (you may need to click "Check for More" or "Show All" to get full list of income items to appear) -> Capital Loss Carryover (see last screenshot)
2. If you are referring to a current year loss from an investment sale, then follow these steps -
You will enter the sale that produced the loss in the investment sales section of TurboTax
Follow these steps -
You can also find investment sales under Federal Taxes > Wages and Income > Investment Income > Stocks, Mutual Funds, Bonds, Other