Very, very, very, very slowly...that is how it is done.
As your cat is a bit older, she will probably resist the new addition, she has been the queen of the domain for some time now and may not appreciate the little intruder.
So in order to make "Bringing home Baby" a smidgen more smooth (be warned...there will be lots of hissing and spitting and crying and whining...this is natural) these are the steps I use when doing introductions:
1) Get a blanket or something along those lines that belongs to the kitten and bring it home. Put it somewhere where your adult cat will be able to smell it and scent it. Leave it out for as long as you can, maybe bring some other kitten scented items and let your cat get used to the smell. Do this as early on as you can.
2) Prepare a spare bedroom with the kitten's supplies, toys, bedding, litter, food, water, scratching post etc the day before you are bringing kitten home and close the door.
3) Bring kitten home and straight into the prepared spare bedroom. Let her out of the carrier to explore this room. Sit and quietly talk to her and let her go at her own pace. When she is ready, show her where the food and litter is. Once she seems somewhat settled, you can leave her on her own for a bit, if it is dark out, I would suggest leaving a lamp (secured) or a nightlight on so you can see her when you go back in and so she can see where she is going better in a strange environment.
Don't forget to go back and check on her several times a day, just sit and play with her for a few minutes. She will be sleeping for a good chunk of her day so don't feel you need to entertain her 24/7.
4) Day 2, play with the kitten and get her smell all over your clothes and hands and then leave her in her room and go play with your cat. If the kitten hasn't been given shots/deworming pills, then make sure you wash your hands before playing with your cat.
Repeat #4 for Day 3 & 4
Day 5, Put up a baby gate or 2 and let them smell eachother through the gate. If your cat is overly stressed out, close the door again but keep playing with the kitten so the adult gets used to the new scent.
Keep trying through the baby gates for a couple of days until your resident cat seems to somewhat accept the new comer
5) Give the resident cat some treats and put her in a room alone away from the kitten. Let the kitten to explore more of her new house. Watch her carefully and if she seems too stressed, back in the room. Try again later.
At all times make sure to warmly reward and treat the resident cat - bringing home a kitten shouldn't be perceived by her as punishment.
Once the kitten seems comfortable with the house (this can take up to a week or two to get to this point) allow supervised visits in the kitchen or living room with the two of them. Expect hissing, growling, paw swats, posturing etc. As long as the resident doesn't start beating on the new one and biting etc just leave them be. They have to peck out the dominance of the house themselves, yet you don't want them to get hurt in the process.
Once they seem to be okay together, leave the room for a bit and see if they behave when you aren't around. Once you feel you can leave them together you can move kitten's food and litter to their permanent homes (again slowly, cats don't like change) and they should be good to be friends.
I have never in 25 years of owning cats had to take a kitten back because the resident cat wouldn't accept them. If you do it slowly enough, they will learn to love or at least tolerate eachother. Then one day you will come home to see them curled up on the couch together ha ha.
Through out the entire process, make sure your cat is getting lots of praise and loving, especially when she is nice and gentle with the kitten. Bribery with treats to make her understand that the kitten is a good thing can work too!
Sometimes, and trust me, this is incredibly rare, they hit it off right off from the start and there is none of this introduction period. My sister had that happen once with her older resident cat and a new kitten. She brought Midnight home and popped her into the sleeping area with Spalding and they have been inseparable ever since (10 years!!). But...that almost never happens.
Good luck and enjoy bringing home kitten!