The legislative body of the municipality shall provide for the manner in which the regulations and restrictions and the boundaries of the districts shall be determined, established and enforced and from time to time amended, supplemented or changed. However, no such regulations, 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 15 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.