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Question: how do you program the via point option

I want to drive to florida using my garmin;since I don't want to follow the gps route and go through New York, How do I program this gps using specific points to force it to trace a route using the main highways I would want it to program.I thought maybe I could use the via point option to forcce it to trace the route I want to use;however the manual is not clear on how to use this option and franckly I'm not shure I'm doing it right. could you guide me ? tank you