Hi there, hope!
Setting boundaries is a topic I'm still working on. Have you read this article here?
https://bpdfamily.com/content/setting-boundariesOne thing that's been pointed out to me is that when you set a boundary, you don't have to tell the other person -- or anyone. It's set within yourself. For instance, in my case, I didn't say to my H "I will no longer tolerate demands that I cut off my family. That's a boundary for me." I just decided that was something I wouldn't tolerate anymore. If he does demand that, I refuse kindly, but firmly and stand my ground. Hasn't come up in a while so I haven't really practiced that one.
You just need to decide what those boundaries are and think through how you'll respond should that boundary be crossed. We can help with that if you'd like to share.