I'm as direct as possible. In business and in my personal life people know me as the person who will give you the absolute truth to anything you ask me even if I know it will hurt your feelings. I often caution people about this when they ask my opinion. If it were me, I would tell them my intent is to set boundaries and what the boundaries are. I would let them know it is my expectation that we both stick to them and I would like feedback if they notice I'm crossing a line as I would do the same for them. Of course, I'd also be open to feedback so that the process of setting the boundaries is a shared effort. I'd explain why it is important to me and why working within the boundaries is beneficial to the both of us. People have an easier time accepting change if they feel they will benefit in some way.

Good luck to you and just remember it is really difficult to get what you want if you are afraid to ask. People can only manipulate you if you allow it once you recognize it. Good for you for taking action.