Lay down the law
Definition 1 of 2
Pro Tip 1/3Assertive Tone
The phrase often suggests the speaker is asserting control or authority in a situation. 
The captain decided to lay down the law before the ship set sail.
Pro Tip 2/3Not Always Negative
Though it may sound strict, it can also mean setting clear, needed guidelines. 
The teacher laid down the law about plagiarism in her class.
Pro Tip 3/3Not Literal Law
The 'law' refers to rules or standards, not actual legal laws. 
The camp counselor had to lay down the law about lights out times.