Buy a simple point-to-point ticket, which will take you from Bern to Milan. It will not allow you to hop on or off the train at various stops, but it will be cheaper than a Railpass.
Book a Railpass if you think you might want to travel to other cities outside of Bern and Milan. The ticket will be more expensive, but the adjustability you get in your schedule just might be worth it.
Purchase your train ticket from Bern to Milan online. Cisalpino (the train company) does not sell tickets on their website but there are a number of other train travel sites that provide such a service.
Select between first class, first class children, second class, second class children or a seat reservation (if you are holding a valid Eurorail pass). Each class carries a different cost. Second class tickets can be almost half the cost of first class.
Decide upon your departure time and date, then enter that information.
Call directly to a railway travel agent should you decide not to book your ticket online. Remember that business hours will vary and that the summer is the busy season--so you might be on hold until you reach an agent.
