The period of adjustment and creation of a new routine can stretch beyond the normal 6 to 12 weeks. If enough factors are in conflict, the cats will become adversaries rather than friends. The reality is that many times choice is not an option. This type of introduction can be rough. Be prepared for a prolonged introduction especially if the cats have lived alone since kittenhood and have no experience living with another cat.
All introductions start with the same basic rules.
If the cat's health record is known and space is severely limited, a large cattery or kennel cage with a blanket over it will suffice. If you don't have a spare room, your bedroom or bathroom can be set up as the isolation room. If your cat is used to sleeping with you, you may have to use the bathroom. If circumstances permit, do this at least two weeks before bringing the newcomer home. The isolation area should be cat proofed and well ventilated.