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 decline a family holiday invitation, use this factual base: acknowledge what was requested, any real timing issue, and whether your answer applies only now or more broadly.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Give a direct no, a limited alternative only if you mean it, and any practical information they still need. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not overexplain private details, blame another person, or say maybe when the answer is no. When pressure, coercion, or retaliation is possible, use a safer channel and involve appropriate support. You do not need the other person to agree with a boundary for you to follow it consistently.