Begin by following the steps in Basic Promotion Setup.
If you selected the "% off order" option for Promotion Type, then under Rules you will enter a percentage. This will be subtracted from the ticket cost only, and will not affect any fees.
If there is no minimum purchase required, and/or no max lifetime discount, leave these 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.