A submissive character or characters are slowly brought to accept a new submissive role. Their decline into submission should be gentle and not obvious. The submissive character might get suspicious about the dominant characters intentions, but usually finds a way to rationalize the behavior. The submissive only recognizes how far things have come when it's too late for them to go back to how things were. For example, in a story about pet play a dominant character might start by giving the submissive character/characters a cute pet name, and then find an excuse for them to sleep on the floor, then find some reason for the submissive to wear a choker/collar, then have them wear less clothing, and continue on like that until the submissive has fully been converted to their role. That previous example can be used for all kinds of Dom/Sub relationships. Just change the tropes to match the relevant relationship in the story. Focus on loss of bathroom privileges, wearing diapers, infantilizing, loss of responsibility, etcetera when writing about ABDL. Focus on loss of privacy, loss of privileges, loss of decision making, etcetera in stories about slaves. The most important thing to do is just to make the descent into submission slow and subtle to the character that is being made submissive. They shouldn't be aware of the submissive role being forced upon them until it's too late to do anything but accept their new life and role.