Do I need/How to install flue damper?

typically a key damper is installed above the stove in a section of stove pipe. I would not put a damper in your liner. There are many possible problems with that but the biggest by far is that you can not take it out easily to clean chimney. Most people do not use or need one so just forget it.

Only needed for long vertical runs where the chimney is a) well insulated from outside cold, b) in constant use thus not cold c)vertical height exceeds a critical threshold for your stove (draft is too high to permit control via the inlet air control on the stove.

My chimney is a 9-10ft run of single wall stovepipe inside my great room (puts out a lot of heat), then meets the class A at the ceiling, followed by another 7-8ft of class A in the attic where it is not quite as cold as outside + the final 3-4ft above the roof line. So I have at least 20-21ft of vertical rise and most of it is in insulated space. The only place I collect a deposit in my flue is the last 4 feet that is out in the weather.

My chimney was an absolute pain until I put in a flue damper and it could stand a second one 6 ft above the first. With the current cold weather and big temperature differential, even with the flue damper fully closed, I still have way more draft than I need on my T5. I bonded a 1/4" washer over the air wash inlet with stove cement to reduce the cross section by about 4x and have ground back the stop on the air inlet control so that it can shut all the way. The thing with excessive draft is that the more you close up the leaks, the harder it pulls on the ones left, like the secondary air inlet. Really, it is far better to leave the stove alone and just choke down the chimney until it behaves. Unfortunately, as mentioned, flue dampers do create service issues. In my case, I don't even run the brush down to the damper anymore, since I know the stovepipe is clean, so the main thing is to be sure the dampers are open when you clean above on the roof.

I did have a bird get through my chimney cap and fall down the flue and that really was a pain to get out....

Open damper fully before opening the stove door, else smoke will pour into the room. Reload the stove, get it cranking, close the primary air some, then when stable fully close the damper. Give the stove time to adjust, then adjust the primary air to the right point. Between the damper and primary air you get to set how much primary vs secondary air you will have. Damper open + primary fully closed = lots of air through the airwash and the normal amount of secondary. Damper closed + primary slightly open = less secondary air. For instance when burning down coals, lots of secondary air is not your friend.

So the answer is, you are going to be working the damper every time you need to open the door to the stove, which is often. If location is a hassle, you could fit it high enough in the masonry chimney that you can drill a hole through the brick to fit an external handle. Of course everything will have to line up just right...

Good luck