Tips for having your cats get along are similar to tips for introducing any new pets to a new household or another pet. Some animals are very flexible and take no time in situating themselves in a new place, while others take a bit longer.
If your cat seems to be having trouble adjusting to the new surroundings or the other cat, you can always place the cat in their carrier or another "special place" meant just for that cat in the room where you spend the most time (door open or closed depending on how scared the cat seems). They often just need to see that you are not abandoning them. It also helps them get an idea of different spaces available. The other cat may stare, but it will likely not get too close.
After a bit of adjustment time, and the cat seems ready, try coaxing the cat out while you put the other way in a different area. They will know they both coexist, but having two separate places at first will help them retain control over their territory. As your new cat gets used to the place, periodically give them time in the same space (with supervision, of course).
This phase may be the longest and constantly include standing, staring and hissing at each other without really moving too much. If it looks like an attack is coming, and it isn't playful, separate them again until they calm down. I don't like using the squirt bottle, but some cats respond to it. If the "playing" is getting too rough, a good squirt may make him/her back down. What I have seen before is once the new cat gains confidence in the surroundings and escape methods, he/she will often become an aggressor as well. This should not be viewed negatively unless real harm is coming, because that is their way of getting along.
However, you can't force your cats to get along. What you may hope for is just a relatively calm household with two separated areas. However, once they get used to each other, and realize that there is enough room/food/litterboxes for both of them, they will typically peacefully coexist. Cats are just like people. They have different personalities, and you can't get along with everyone all the time.