Definitely look around on this site for tips on introducing cats. The article will tell you exactly what I would...
Put them in separate rooms, new cat in a room with old (already-owned, rather) cat having run of the house. This helps to follow the social structure cats so badly feel the need to follow. Make sure, of course, they each have their necessities...food, water, litterbox, toys, etc.
When you see them starting to play paws under the door, you can let your new baby out to sniff noses, and allow each of them to check each other out. Be sure to be right there in case they start acting aggressively. Don't react too hugely if your current babe decides to bat the other in the head, or some such. It's a natural thing they will most likely have to go through, and it's the establishment of the social structure. They might growl or hiss, which is also normal. But if you see them getting ready for an all-out fight, intervene and put your new baby back in the room for a day or two.
Anyway, if you're sure to do things in their time, are sure to be patient, and are ready to give your current baby loads of attention and love to be sure he/she knows you aren't replacing him/her. Be sure to give your current friend lots of exclusive time with you to make him/her more comfortable.
Otherwise, it should go pretty smoothly. Keep in mind, if they take to each other right away, your current kitty might be fine for a week, maybe two, and then have about a week of just NOT putting up with your new baby. It's just that your current friend thought it was a temporary situation, and has then realized it isn't, and is upset. It'll pass. Like I said, just be patient.
Also be sure to find that article here on the site.
Let us know how it goes, ok?