Hi, Michelle and congratulations on your new, furry addition, Lizzie.
Yes, you are right, new introductions must be made very gradually, so the cats get used to each other slowly. One method I like is the scent transfer one. Simply take a slightly dampened cloth or paper towel and pet one cat, then the other, then vice versa; keep doing this frequently throughout the day. This will deposit one cat's scent on the other's coat, so they smell each other all the time.
Also, get a Feliway plug in diffuswer; this emits calming cat pheromones and is excellent in new introductions:
These sites, below, will help you with more tips and techniques for new cat introductions. For the moment, I would keep them separated until you start from square one with the introduction process.
Remember to praise a lot and give healthy cat treats, when they behave well, together.
She's being aggressive with him because she's afraid and adapting to a new environment and a new cat.
I hope things improve soon, and keep in mind that this is a gradual process which may take a little while.
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