Direct Answer: To communicate a boundary without fighting, use the four-step formula: 1) State the observable fact ('When you do X'), 2) Share your internal limit ('I feel/need Y'), 3) State your request or agreement ('I request Z'), and 4) Provide your clear next step ('If X continues, I will take action A'). This keeps the focus on your choices and avoids accusatory 'You' statements.
To construct a bulletproof, non-defensive boundary statement, assemble these four pieces together like blocks. Do not omit the fourth step—it is what turns a request into a boundary.