How to audit custom events?

You have system events and application events. System events are events recognized by Content Server (e.g. checkin, destroy). You can turn them on or off and when they occur, the Content Server will take care of creating a dm_audittrail object. Application events (or custom events if you prefer) are application-specific. It is the responsibiilty of the application to create them. You can do that with the DFC method I pointed out. That code can be inside a TBO, SBO, directly in your webapp It doesn't really matter but it's your job to call that method with the appropriate parameters.