When introducing cats, you might want to think about the existing cat and how it will respond to the new cat. The new cat will deal, but the existing one will have some problems.
First off, your cat thinks it is being replaced in your affections. Doubly so if the new cat is a kitten - or just smaller. This makes your cat insecure, and it may have problems accepting the new cat. Your cat that formerly had fastidious litter habits may begin to soil all over the house in frustration, making everyone miserable.
You will need to add a litter box at the beginning, whether your cats decide to share or not. Don't expect your new cat to be comfortable using the existing litter box, and don't expect your existing cat to lightly share his private potty!
Bonding play is extremely important, too. Get your existing cat used to play and then offer the toys to the new cat. Usually, this is the way things need to be done, but sometimes, you might offer the toy to the new cat first to get the scent on it. Another way to get started is to get each cat their own toy and then a new one for joint play. After your cats are playing individually, you can begin to help them bond as a team.
Don't forget that love and cuddles are your best training tools when introducing cats. Your existing cat needs a lot of love right now. Later, you can cuddle the new cat in front of your existing cat, but at the beginning, don't you dare!
The "scoop and smooch" is a great tactic to help your cat accept the new littermate. Be sure to do this each and every time you come in from being outside. More on that, later.
The existing cat may have other problems as well. If the new cat wants to play all the time and your baby isn't used to that, your existing cat may feel that s/he is being attacked all the time. Time-outs are very important in this situation. If the new cat can't or won't leave the existing cat alone to potty, sleep or eat, put a belled collar on the new cat. If nothing else, it will give your elder statesman warning when the new cat is coming, and perhaps time to prepare or hide.