Begin by following the steps in Basic Promotion Setup.
If you selected the "$ off tickets" option for Promotion Type, then under Rules you will enter a dollar amount. This amount will subtract from the price of each individual ticket that matches the criteria.
You can apply this to all ticket types, or selected ticket types.
If there is no max discount per order, and/or no max lifetime discount, leave those fields blank.
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 discounted orders 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 non-discounted tickets on matching orders.)