a minimum of 6 months in a different room (ideally a different building). Separate feeding forceps, water bowls, equipment, etc. Allow not contact between the new animal and the established. Insure you thoroughly wash your hands when moving between the two groups. Never offer a refused food from one to another. House on paper towels to allow you to determine any defecation issues or the presence of external or internal parasites.
Pretty much a straight forward process.
Warren