Sooner of later most pet parents will want to adopt a furry friend for their current furry companion, or just add one to your "family". If this is you, it will cause stress to your present companion. It is easiest to introduce a female kitten to an established cat. However, if you have a senior cat, you should wait until it has passed before trying to introduce an additional cat.
The first step is to consider your current cat's personality and then visit your local no-kill animal rescue/shelter. A rescue that uses "foster homes" will often know more about their cat's personalities and should be able to make a more intelligent match to your present fur buddy. This is not to say other shelters are unable to help. However, often they have not had the intimate one-on-one interactions as "foster homes".
Once you're new fur buddy is home, there are some steps that will speed your ability to introduce him or her to your other cat(s). First, select a room where your new fur buddy can reside. Often, a second bathroom or room with a hard surface floor, just in case there is an "accident" due to the new environment. Whichever room is selected it should include; fresh water, food, litter box and of course a toy or two and some type of cat furniture to climb and hide within.
It is important to spend quality one-on-one time with your new friend. This includes, playing, brushing and calm conversations. Of course your other cat will want to know what is going on behind closed doors. Observe your new cat and see when he or she shows interest in exploring the room beyond. When this happens, allow your new fur buddy to explore and interact with the other resident cat(s) on a limited basis. Watch to see what happens. Does your resident cat accept the newbie or growl and hiss? Grooming the new cat is a great sign, but don't assume that all will be well all the time. Continue the limited interactions until there are no signs of dislike.
With time and patience, you will have a wonderful fur buddy for your resident cat(s). Provide praise, treats and reassurance that the resident cat(s) will always be number one in the house. With this you have just succeeded! ENJOY! As always, don't forget to look for a new piece of cat furniture for your new resident.