Steps
Step 1
First opened a small area of the plasterboard that I was confident to close in case it turned out to be complex one.

Step 2
Once the small area is opened assess load bearing of the studs and create necessary space to place door frame with structural pine. Double and triple do your math to include door opening, door jamb spaces as needed. Plus add 10 mm to be safe.

Step 3
Add door frame and door jamb. Cross check measurements to be sure that you have enough space for door plus some offset

Step 4
Add door and finish the inside with plasterboard and MDF

Step 5
Finish plasterboard with jointing compound and tape and sand (this is where I got lazy). Install skirting boards. use wood filler close all holes and then paint
Step 6
Install smooth edge along the perimeter and lay down carpet. Install shelves.
