after the rebellion, snowball and napoleon reveal the seven commandments. where do they get these rules and why do you think they paint them on the side of the barn?

The Pigs make up the rules by telling the rest of the animals they've spent three months reducing the principles of animalism into the 7 commandments.

The commandments were paintede on the side of the barn in large letters so that everyone could see them. They couoldn't all read them, but they could see them.