Clay liners are typically just stacked and laid in place while brick goes up surrounding them. I’ve done a few stainless liners- the upper termination just sits on top of existing chimney. The pain in the ass part is getting into the side of the flue down low- you need a good hammer drill with chisels to slope the flex liner up and to mortar around the liner, setting a termination ring in place, you need your wye fitting to the two appliances to be at a safe height above both appliances and you should not use appliance with powered vent when the other is natural draft- ask a plumber if they will let you snake the liner and have them do everything in the basement.