Begin by following the steps in Basic Promotion Setup.
If you selected the "Comp Tickets" option for Promotion Type, then under Rules you will follow the prompts to specify how the promotion works. Use the "add another rule" button to apply more than one rule.
You can even let customers enter a code for a free ticket, with no purchase required. Just enter zero in the second field, and make sure the box is checked at the top of the page labelled "allow free orders." (See the screenshots below for reference.)
Once finished, click Add.
Note: Please test the promotion using the customer order form, and try all possible scenarios. Make sure that you proceed at least to Payment, to confirm that the discount is working as expected.
You can see data about comp tickets using the Discounts Report, the "coupon" filter on the Order Search page, and the coupon_code column in order exports.
(Note: These results include orders where the customer entered a code but didn't use it, and they include any paid tickets on matching orders.)