Cut away any flue tile that juts above the chimney top, leaving the metal flue liner in place. If you fail to do this, water can get into the crack between the tile and the chimney pot and cause damage. If this happens in freezing weather, ice will form and cause cracking.
Spread a ring of mortar three inches wide around the outside of the flue. While it is still wet, set the chimney pot into the mortar and press down so the bottom edge is sealed.
Trowel the mortar so that it slants away from the bottom of the chimney pot. This channels rain away from the chimney pot base so water doesn't seep in. Let the mortar dry.