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 apologize for waking someone up, use this factual base: name the exact action, the inconvenience it caused, and what you understand now.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer the smallest useful repair and give a realistic time for completing it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not inflate the apology, fish for reassurance, or demand immediate forgiveness. If the person has asked for no contact, repair what you can without sending another message. You do not need the other person to agree with a boundary for you to follow it consistently.