Assassin's Creed Valhalla

There are a total of 45 Order members that must be killed for this achievement.
 
Of that 45, 15 of them are the Zealots. There is a choice you can make early in the game that will either cause the Zealots to hunt you or not, and if it's the latter, you'll need to track them down yourself. The former method makes this much faster, but if you chose the latter choice, then a link to the locations of all Zealots can be found HERE. Do note that many of the Zealots do not have fixed locations, and instead roam around certain territories, which can make them difficult to track down. I highly recommend going through all of the collectibles on the map of England (wealth, mysteries, and artifacts) first. That way, you'll almost assuredly stumble across most or even all of the Zealots in the process of doing that, eliminating the need to hunt them down. But, the link above can still be used for guidance of approximately where to find each. I do want to point out that for Cudberct, for me, he was not located anywhere near where the guide shows. For me, I fast traveled to the western viewpoint in Lunden, and from there went directly north around 300-400m and found him. Likewise for Cola, for me he was in Ledecestrescire near the north end, not in Lincolnscire or Eurvicscire as the guide says. Everyone else was where the guide said, though.
 
Of the remaining 30 actual Order members, 12 of them are killed during story missions (3 from each of the 4 branches). You'll kill 3 in the beginning of the game, 3 in Lunden, 3 in Jorvik, and another 3 in Wincestre. Building the Hidden Ones Bureau in your settlement will reveal the remaining two Order members from the rightmost tree. You can then simply track them on your map to kill them easily. The remaining 2 Order members in each of the remaining 3 branches need to be located. This can be done by finding clues to their whereabouts in the game, or you can use THIS GUIDE to see where each Order member is. Note that, if you don't find all of the clues for an Order member to reveal them first, they are essentially civilians, meaning Odin's Sight won't turn them red, and there's no way to see that you have the correct person until you kill them, at which point you'll get the prompt to confirm your kill. Even so, most Order members are isolated enough that it is pretty obvious who they are, and there isn't really any penalty for accidentally killing a civilian anyway.
 
With all four branches of Order members killed, you will have revealed all 5 of the final tier of Order members. The 4 on the left can all be tracked on your map and killed like other Order members. The rightmost one can only be killed by returning to Hytham in the Hidden Ones Bureau in your settlement. After talking to him, you'll be given a new quest line that allows you to travel to Vinland to track down this Order member. Do so and follow the short questline to kill him.
 
Once all 15 Zealots are dead and all other Order members are dead too, you'll also need to complete the main story arcs. Specifically, you'll need to complete all England pledges, the Hordafylke pledge, and the Hamtunscire pledge. Once all of that is done, return to your settlement and head up to your longhouse. Hytham should be waiting for you out front, where he'll give you a letter from someone you've been in contact with. Follow this short questline to find the final leader of the Order, eliminating him to unlock this achievement.