Trip notes work like event notes: create a notes form, associate it with a trip, and use it to capture information about the trip as a whole rather than individual events.
To add a notes form to a trip:
Create a new form with the Form/Event Notes scope, as described in Form and Event Notes.
Go to Events, then Trips.
Select the trip that should use the new notes form.
Select Edit in the top right.
In the popup window, select your new notes form from the Notes Form list.
Save.
Refresh the page, and the Notes button will appear in the top right-hand corner of the page.
Click Notes to fill out the notes form for this trip.