Turbotax makes that easy. You enter her at the beginning of your tax return on the Personal Info Summary as always (if you used Turbotax last year, her info should already be there). The software will recognize from her birthdate that she is over 18, and will ask the appropriate questions about income and support to determine if she qualifies.
When you get to the Deductions and Credits page, under You and Your Family, you should see Child and Other Dependent Tax Credits. This is the same place you used to qualify for the Child Tax Credit, but will in this case calculate that you are eligible for the $500 Other Dependent Credit.
