Room renter threatening me and accusing me of following her with my eyes...

In CA we're not allowed to record without both parties consent. Can I disclose that I will record our conversation should she try to bother me again? Do I need consent or can I just give her the knowledge I am recording?

You need consent. In other words, take out your recorder, turn it on and ask "Do I have your consent to record this conversation?" "Yes" and you're good to go. "No" and you turn it off and say "OK, then stop talking to me and put your issue to me in writing and I will respond in writing."

In the future anything you have to say to her should also be in writing.