We have three rescue cats, all got at different times.
We kept the newbie in a room of its own with a litter tray, scratching post and toys. The other (or others) used to sit outside the room, but we didn't let them see each other for 1-2 days. Then we let the newbie explore, whilst keeping a close eye on the other(s). Often, the current kittens would be more interested in sniffing in the new room before even spotting the newbie! I was also told that petting each one, then petting the other helps to get the scent spread round.
Good luck, and just take it nice and slow. We had a few spats, and still do, but they otherwise muddle along together nicely.