You do this within the Attendee record:
1. Navigate to the Attendee's record.
2. Open the module you want to appear first in their Itinerary (Function, Accommodation, Flights, etc.) and click on the Itinerary Tab.
3. Set the ranking order you want for that module (e.g. 1 will appear first, 2 will appear second etc.)
4. Do the same for each of the modules you'd like to appear in a particular order.