Map every part of the trip
Mark each connection as well as the final stop. For every country on the route, look at the current entry or transit rules that may apply to the traveller.
Check passport validity requirements and any visa or other authorization. Confirm uncertain details with the relevant authorities rather than relying on a broad rule of thumb.
Return to the map
If a flight or destination changes, review the new route. Keep booking and contact details close enough to use while travelling.
This article offers general information. Requirements depend on the traveller and itinerary; confirm current rules with the appropriate authorities.