Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to exchange a child's belongings with a co-parent, use this factual base: list the specific item, ownership, condition, current location, and any real deadline.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a neutral handoff, tracked delivery, or authorized third party and confirm when the matter is closed. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not enter property, withhold unrelated items, attach emotional conditions, or arrive without agreement. Use a public place, third party, workplace process, or legal support when ownership or personal safety is disputed. You do not need the other person to agree with a boundary for you to follow it consistently.