Introducing a new puppy into a home with an aggressive dog?

We are getting a new puppy. The puppy is a Rat Terrier and is four months old. We already have a three year old dog in the house named Roxy. Roxy is a boxer-lab mix and I am very worried that she might attack the other dog. In the past Roxy would attack other dogs that were in the house or in the yard. However she is fine at the kennel or the dog park. How can we introduce the two dogs without Roxy attacking the new puppy? Thanks for your help!