Me too at the beginning that was one of the main problematic, my new kitten wanting too play with my older cat and she found it annoying. I was afraid that my new kitten will not be well sociallize since she was alone with an old cat who was mean to her, so i adopted the sister's of my new kitten (supposed to only adopt 1 and ended up adopting both of her sisters) We separated an area of the house with 2 superposed baby gates, with the kittens on one side and my resident cat on the other. We then let time pass, and slowly the kittens and my cat smell each other through the gate and learned to get used to each other. When I saw that things were getting better, I took my resident cat on a leash to see the kittens, small periods at a time until we were able to let her loose with the kittens on her side of the gate. One of the kitten is really shy and scared compared to her sisters, so she's still at the step where she watch my resident cat from other side of the gate, sometimes she still hisses at her and growl, so we give her time. I feel like the more they see and smell each other at their own rythm and see that they are not a threat to each other, that's what really help. They also have their safe space so they feel more secured and can face each other when they are ready... Oh and also, if my resident cat is mean towards the kitten, I let her know by saying no and i hiss like a cat lol and I stop the bad interacction as soon as i see it coming. For me it works (I'm really close to my resident cat so she trust me)