I did not research introducing cats to each other before i put my newly adoped kitten and my one yea

I made the mistake of not following the steps of introducing two cats.  I adopted my 2 month kitten, Mia, yesterday and brought her home where she was face-to-face with my one year old cat, Molly.  Molly, who has never been around another cat since her adoption (at 2 months) was instantly on guard and followed Mia around everywhere trying to assert her dominance.  The cats would try to swat at each other, and keep chasing each other around.  

Now that there has been negative aggression associated between the two, how can I improve the situation. Should I now follow the steps of introducing two cats  (keeping them in separate rooms, familiarizing scent, etc) or let them fight it out? They've both been freely roaming the housing chasing each other, constantly swating and play biting for the past 12 hours.  There are occasional hisses and a few growls.  

Or is it too late? did I miss up on their introduction so bad that they'll always be on a bad foot with each other? 

It doesn't seem to be that bad. As a matter of fact, they're on the way to becoming friends.   You can do some scent exchanges to hurry the process. 

I did the same thing yesterday! Except I have two cats, one male and one female. A year ago, when I brought my male cat home the same way, my first cat only hissed a bit but I guess he was enough of a scaredy-cat and coward that my year old girl adjusted quickly (she's pretty dominant, and this kitten is very assertive too).

I just keep a really close eye on them, and my kitten knows to run to me if she's getting in too deep or is tuckered out. I watch them hiss and swat at the kitten (and she growls and swats right back, even though they outweigh her ten to one!) And make sure she doesn't have cuts on her. My cats aren't using their claws against the kitten. It's yours? If not then it's probably like yayi said, they're on their way to becoming friends. Hopefully it doesn't take too long!

i just went thru the same thing, same ages of cats too.I sadly could not do the proper introduction due to living in a studio apartment. Its only been less than a week. It took my cats atleast 3 weeks. If anything if you do not see improvements start all over. If you do have room separate them for a while let your first kitty roam her turff for a few hours alone also i wouldn't leave them in the same room over night. i had put my kitten in the bathroom. It will get better.Its so early.

We  just brought a kitten home and did the same thing.  Just dropped her in the house like a bag of potatoes. Our cat didn't really like this kitten at first, she was little and weird and annoying..... now he doesn't mind her.  She's too small to play with now and is  leaving in September with to go to her new home. 

When we adopted Juli  two years ago, Junior hated her, now they are best friends and some people think they are a married couple.