One big thing is not to take away anything your first cat is used to getting because he/she may associate that with the new cat and resent it. So much of it also has to do with scent. Your cat has to get used to the new cat's scent before anything else. If you throw them together before one or the other is really ready, they may never end up getting along.
At first I let them be together and then Itook your guys' advice and the new kitty is in the bathroom. He cries and I feel bad for him. Theere were no fights when they were together...my older cat just growled an hisssed whenever the new one came over. I haven't taken anything away from the older kitty, but I have been giving him a little wet food once per day (he usually only get dry) When do you suggest I let the new kitty out again?