There are many factors to consider. Without knowing the boundary issues and the nature of your relationship, it is hard to say precisely. I do know that you need to be clear and direct. Avoid noncommittal language such as, "I wish you would...," or "I don't like when you...." This does not give any direction.
Instead, you need to say very firm, direct statements such as, "Do not do...," and "Please do...."
If you are soft natured like me, you may find that you naturally are soft in not only how you say things, but the words you use. You need to make sure your words are very clear. If there is any ambiguity, then someone who has boundary issues will take advantage of that.