How to level and smooth a room wall

Good evening,
I want to renovate a room in an old building.
The walls look like in the pictures. The plaster is peeling off in many places, leaving a surface with green paint.
In the end, I want to wallpaper the wall with wood fiber wallpaper.

My question: How do I best treat the wall (and if possible without much effort), so that the wall looks nice with wallpaper in the end and no unevenness etc. can be seen?

Thank you very much in advance.

Everything that flakes off, brush/scratch off well, prime and replaster, then sand and wallpaper.

Get yourself a large scraper with a long handle and then go ahead. If everything is really loose and off, clean well with water and a brush. Then fill the holes and wallpaper. Have fun. The blisters on your hands will go away, the room will stay nice for a while. It's worth it :-)