Whispering Willows

After you and have gone through the basement with spiders mentioned in it you should be in a new area. The first room you're in should have the fireplace. Click on it and achievement.

During Chapter IV, you'll descend into the secret door in the observatory, and after the steam room, you'll climb a ladder into a room with a fire pit near the door. Interact with the bonfire to gain the achievement.

It can be seen in this 100% walkthrough at 48:57.

Using the above video will get you ALL the achievements in the game in about an hour.

The walkthrough on here, the flags and most players tell you this achievement is missable.
Don't be fooled by that if you don't know the game yet or if you missed this.
If you complete the game and miss this one achievement, load your game again and you will be in control before you hit the end of the game. Turn back and backtrack a bit to find the fireplace and unlock the achievement this time.
I can't really recall the way I went and where I was put after reloading, or if there was a shorter way through unlocked doors than going back the story path.
If you want to be safe and not wander about
- go back through the hall where the knights are
- upstairs
- climb up the ladder
- pass the spiders going right (try and listen to them walking back and forth if you are having trouble)
- climb down the ladder
- go downstairs through the room with instruments
- downstairs
- right
You should find yourself in the room with the fireplace. Stand in front of it and hit

or refer to a walkthrough video, e.g. the one in Maka91's solution, to find the way by reversing the way they go.
I actually found it first try within less than a minute and I'm sure you can do so as well because I didn't pay much attention to how the mansion is built for orientation.

The other missable achievements might still be missable as you can't go back that far but I haven't tested that, honestly.