How to insert a chimney flue?

I presume you mean a flue liner. The suggestion to push it down from the top is wrong, you will struggle on a roof with a 30 foot coil of aluminum liner. Take a good strong rope and attach a weight to it, drop it down the chimney pot from the roof down to the fireplace you want to line. At the fireplace end, remove the weight and tie the rope securely to one end of the liner and with an istant below to uncoil the liner, pull the flue liner up through the chimney from the roof.