How is the best way to introduce a new guinea pig to its new cage mate? We've got a 3 yr. old male.?

We bought another male to replace our sweet Hershey that died about 2 months ago. Hershey and Shadow were together for about 3 years. Right now I have Shadow and the new male in separate cages that are right next to each other. Whenever I put them in the same cage there is a lot of teeth chattering going on - especially by Shadow. How do I get Shadow to accept this new guinea pig friend?