This isn't in any particular order except your first airship ride which is in fact Rabanastre to Nalbina. You will receive the letter after talking to the chief steward on your first airship ride. Here is a list of connecting flights:

Rabanastre to Nalbina
Rabanastre to Archades
Rabanastre to Bhujerba
Bhujerba to Balfonheim
Balfonheim to Nalbina
Balfonheim to Archades
Archades to Nalbina