The legislative body of such municipality shall provide for the manner in which such regulations and restrictions and the boundaries of such districts shall be determined, established, and enforced, and from time to time amended, supplemented, or changed. However, no such regulation, restriction, or boundary shall become effective until after a public hearing in relation thereto, at which parties in interest and citizens shall have an opportunity to be heard. At least fifteen days’ notice of the time and place of such hearing shall be published in an official paper or a paper of general circulation in such municipality.