How do I create a new trigger?
The first step is to read the Trigger Guide and Trigger Reference sections of the readme.html that is included with the mod.

Among other things, the guide explains how triggers work, what the various components of the trigger system are, and provides example triggers for common categories. The reference section provides specific information about the various components such as the available substitution codes, main events, conditions, and exceptions.

Once you've read those sections, we can continue on to the process for creating triggers.

We will be creating a trigger that notifies us when Polymorph breaks from a hostile unit when we are playing on a mage in the arena.
  1. Since a new trigger is being created, click the Add New Trigger button, enter "Poly Broke - %r!" for the output message, and click Okay.
  2. The trigger conditions configuration dialog for the new trigger will automatically be shown. You can click the trigger conditions icon button (the gear) next to a trigger to access these settings manually at a later time.
  3. Click the edit Trigger Classes icon button and uncheck "All" classes. Check Mage for the class the trigger will apply to. It is better to select the specific classes the trigger applies to instead of selecting all classes since by specifying classes, the mod can completely ignore the trigger and save resources for the classes to which it does not apply.
  4. Click the Add Event button next to the Main Events label to add a new main event.
  5. Select Aura Removal for the Main Event.
  6. Modify the first condition to "Recipient Unit Reaction - Is Equal To - Hostile" because we don't want to see when poly fades from our teammates.
  7. Modify the second condition to "Skill Name - Is Equal To - Polymorph". It is case-sensitive, so make sure you use a capital P.
  8. Click the Save button to save the new main event and its conditions.
  9. Click the Add Exception button next to the Trigger Exceptions label to add a new exception.
  10. Select "Zone Type - Is Not Equal To - Arena" for the Exception. This makes it so the trigger will not fire when we aren't in the arena.
  11. Click the Save button to save the new exception.
  12. Click the Save button on the trigger frame to save the trigger's new conditions.

The %r used in step a is a substitution code for the name of the unit that the debuff (polymorph in this case) faded from. You already knew that though because you read the Trigger Guide and Trigger Reference!

The color, font settings, sticky state, sound, and output scroll area for the trigger can be set like any other event.