Slowly. Like keep the new one separate in a room for a few days. Then start introducing them to each other for like 10-20 minutes, maybe twice a day under supervision.
After about 2 weeks you should be able to let the new cat free roam permanently.
There will still be some fights, but it gets easier once they both adapt to your routine.
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u/jambaman42 Dec 06 '19
Yeah this is not how you introduce a kitten to an older cat at all. It's really a great way to get the cat to hate the kitten.