A method, and apparatus for performing the method, is provided for the purpose of forming and inserting seal pieces into a container cap of a type which comprises an integral cup-like member having a top panel, a skirt depending from the top panel, and a plurality of cap lugs formed in the rim and extending partially inward from the rim. A supply strip of flexible seal material is directed into proximity to the inserting stations and precisely cut seal pieces from the strip are inserted into the caps.