Okay, here's how I do it all the time.
Put your new cat in a separate room for a few days before introducing them, preferably somewhere your other cat can't see him/her.
After a few days, put both cats in one room, but put a small gate in between them. They only get to sniff one another. Separate them for another day, then try it without the gate. If things get serious(Serious fighting, snarling, hissing, etc.) get them away from one another immediately and try again later on. Keep doing this until they are comfortable with one another. Then, get them engaged in playing a game with a toy or something. This 'bonding time' will make them good friends ^^ After that they'll be okay, as long as they aren't trying to murder each other every seven seconds.