We have a doorframe into the kitchen that we plan to expand in all directions except downwards (!). 
The idea is to remove the doorframe and expand the opening a few decimeters to the left, where the opening will stretch from floor to ceiling, and the same to the right but opening up over a meter.
A new ceiling is to be installed on both sides of this opening, so it doesn't matter if it looks perfect with the existing ceiling. It also doesn't matter if it's advantageous to divide the ceiling in the opening with a molding or similar.
Does anyone have tips on the approach and construction techniques?
How do you finish the new walls? Standard studs left/right/top covered in plasterboard?
The idea is to remove the doorframe and expand the opening a few decimeters to the left, where the opening will stretch from floor to ceiling, and the same to the right but opening up over a meter.
A new ceiling is to be installed on both sides of this opening, so it doesn't matter if it looks perfect with the existing ceiling. It also doesn't matter if it's advantageous to divide the ceiling in the opening with a molding or similar.
Does anyone have tips on the approach and construction techniques?
How do you finish the new walls? Standard studs left/right/top covered in plasterboard?
The first thing you need to do is check whether it is a load-bearing wall or not. If it's load-bearing, you'll need to contact a structural engineer who can "design" the door opening; additionally, you'll need to make a construction notification.
If it is not load-bearing, then you can just start sawing and prying with a crowbar. Just try to make sure there is no electrical wiring where you plan to saw.
Then you add a new stud in the wall if needed (it might be that there is already one where you want to end).
To make it look nice, you arrange some kind of finer plank or similar to cover the wall end, and for example, put a door trim around it to conceal the joint between the plasterboard and this board.
We did a similar covering around a door opening in our previous house. I ordered a number of strips about 120mm wide, made of thick MDF board, and had the edge milled so it was rounded, about a 15mm curve radius. Painted this, and it looked nice.
If it is not load-bearing, then you can just start sawing and prying with a crowbar. Just try to make sure there is no electrical wiring where you plan to saw.
Then you add a new stud in the wall if needed (it might be that there is already one where you want to end).
To make it look nice, you arrange some kind of finer plank or similar to cover the wall end, and for example, put a door trim around it to conceal the joint between the plasterboard and this board.
We did a similar covering around a door opening in our previous house. I ordered a number of strips about 120mm wide, made of thick MDF board, and had the edge milled so it was rounded, about a 15mm curve radius. Painted this, and it looked nice.
Depends a lot on how the wall is constructed. Light concrete, studs + drywall, plank... ?