Setting a boundary takes one sentence. Keeping it takes every day after.

Announcing a limit is the easy half and the half that gets all the attention. The work is in the small moments afterwards where holding it is inconvenient, where the other person tests it gently, where you could let one slide and nobody would say anything. A limit that is not maintained teaches people the announcement was decoration.

Expect to be tested, and decide in advance what you will do about the first push. Keep the response short and repeat it without new arguments, because explaining again invites a debate. Consistency, not force, is what makes it eventually stop being tested.