1. Get a map.
2. Look for pinch points in your route: border crossings, bridges etc.
3. Look for areas to avoid (big cities)
4. Look for areas to ride (pretty /rural/mountainous)
5. Plan a provisional route in the general direction.
6. Fine tune each days ride. You an do this as you go along, you dont need to start with a complete route.
7. If you are riding to a timetable, have a bug-out plan usually involving rail or bus travel.

My preference is to deal with major cities by commuter rail services rather than struggle to find a bike route out.
Plan a weekend shakedown tour to test your kit, there are plenty of packing lists.