Hi Mom,
This link may give you some ideas
https://bpdfamily.com/content/setting-boundariesThings to keep in mind though:
1. Lots of trial and error has to occur and don't feel bad if things don't work the first time around. It's all case by case basis.
2. Does your adult child live with you?
3. As much as you can , please remember you won't be able to change your BPD adult.
4. Figure out what YOU need, think about consequences to behavior and if you can stick with the consequences .
You are not alone.