How do you enable a built-in variable in Tag Manager?

To enable a built-in variable in Google Tag Manager, you navigate to the "Variables" section in the left-hand navigation pane. Under the "Built-In Variables" heading, you simply check the box next to the variable you wish to enable. This makes the variable available for use in your tags, triggers, and other variables. You do not publish built-in variables; they are enabled directly within the interface. Creating a user-defined variable is for custom data, not for enabling existing built-in variables. Unchecking a built-in variable would disable it, not enable it.