Sales from Square would be entered along with your self-employment business income.  Your income and expenses for your business will be reported on Schedule C of your federal tax return.  

To find the place to enter your sales from Square within TurboTax Self Employed, go to your Federal Return and enter "Schedule C" in the search field at the top right of the screen.  Then select "Jump to Schedule C" and you will be brought to the income and expense section of your business.  

Select that you want to EDIT or ADD income to that business. This will bring you to a screen where you can enter Additional Income that you received from a 1099-MISC, 1099-G, cash, checks, 1099-K or, as you need to do, report your sales from Square.  In the Type of Income Field, enter "Square" and enter your sales dollars in the Amount Field.