Window installation and reducing reveal

I can glue the panel myself, but in this case, it’s a botch. The old window has to be removed because it has already been modified by my father-in-law and one sash no longer holds properly. The new window is already there as well. I am not taking away any light; the window will be about 4-50 cm narrower, but in another spot, an 80 cm door with glass will be installed. So the question remains, how do I do it right?

I can glue the foil myself, but in this case it's botched.

Well, it was meant to serve as an idea to conceal the makeshift construction without high costs, especially since you mentioned a tight budget. It was not mentioned that there is already a window in place. I would advise you to have the window installed by a professional; they will also tell you which preparatory work they would prefer, so it becomes easier for them. Every other word in your text is "botched", so with this manageable topic, I would let the professional handle it. On construction sites, there are always plenty of trades where one can save a lot of money through diligence and effort without creating the risk of a self-inflicted construction defect.

Do you want to make both sides or just one side narrower?

So you could use aerated concrete blocks. You have to cut out the existing wall to then insert the new stones there.
But you can also build with facing bricks. That is probably easier since the existing wall is already built like that.

In any case, you have to create a proper connection to the old masonry. Simply building onto it won’t work.

The window tape and compression tape are self-adhesive, the screw length must be measured. It depends on the window and the wall.
You can also glue the tapes with Sikaflex. Then it will hold properly.