How do I do this??

[DorisDoris]

The idea was to plaster around the doorway. But unfortunately, the concrete part on the left is not level with the door frame, so plastering isn't possible. Level difference is around 3-4 mm.
So what on earth do I do here to make it look nice?

 

[mexitegel]

Adjust with, for example, masonite strips to achieve the correct level.

 

[DorisDoris]

Adjust with for example masonite strips to get the right level.

Should add that I live in Finland and don't know if we have that here...

 

[RapidSledge]

I should add that I live in Finland and I don't know if we have stuff like this...

Buy a masonite board and make your strips yourself.

 

[fgo]

It can be like this (Can't speak Finnish) https://www.k-rauta.fi/rautakauppa/...alevy-rakentajan-laatu-2-7x1220x2745mm-3-35m2

 

[DorisDoris]

Buy a masonite board and make your strips yourself.

Yes, we do have such a board. But now to question no. 2. Are you allowed today to attach one so close to the chimney? Flammable...

 

[Slarvpellen]

Apply a thick layer of plaster where you need to build up, comb it out a bit.. press your plasterboards so they are reasonably straight and nice.. a few screws in appropriate places to hold where needed. If it goes down to around 2mm, you might consider just placing the plasterboard directly over and letting it absorb the unevenness.