Start with: “When you do that, I feel this” (or “a part of me feels this”)
When you do that. When describing the other person’s behavior do it objectively or in terms they can understand and agree with, rather than interpreting their behavior, judging it, or talking about their feelings or underlying issues. This is so they understand what you are talking about, and they aren’t hurt or offended.
A part of me feels this. When describing your feelings, be clear about the difference between feelings and interpretations. You can include both, but be clear about which is which. Your interpretation will often determine your feeling response and what parts get activated.
Make sure not to leave out your emotional reaction, and take responsibility for that.
Speak for your part, not as your part. Speaking for a part means being in Self and talking about what the part is feeling from a centered place. It is especially problematic to speak as a part that is angry or judgmental, which usually means dumping this on the other person.