1: Take old ferret out of cage, and put new one in. This will familiarize the new ferret with the old ferrets smell.
2: After a while, take out the ferret, and put the old one back in to smell it.
3: Let them smell each other through the cage wires, or in a safe place while holding onto them both.
4: If all is well, let them play together in an enviroment unfamiliar to both ferrets.
5: If all is still well, then your done!