All is done quite simple.
1.) A basic skeleton is created from wood using screws through the drywall to the wall studs.
2.) Then a next layer is created fixing it on the skeleton and in the ceiling
3.) The whole skeleton is then covered with a thin plates of plywood
4.) The bottom of overhead is also covered, but I make the bottoms removable so later I can add inset lights.
5.) Then I filled the gaps, sand it and paint it with the burnt sienna like the rest of walls.
6.) The next thing is to create a hidden lights. I used small triangles to which I will fix the tube lights (the flexible transparent tube with many small lights inside)

First I painted with blue also few inches from top on the overhead.
Then I put an Aluminium tape on top of triangles around the whole length to make sure no light will go down.
Then I fixed the flexible tube light which I bought in home and garden store.

The flexible lights have a connector on both sides so they can be connected one after another, creating the length you need. Also it is possible to cut the tube at the end.

Because the ceiling tube light is there more to create feel of big space not a fully light the room (you can even watch a movie while it is on) so a small 24V spot reflectors were attached to the bottom of the overhead, 3 on each side. This creates enough light even for reading.

It is very rewarding to see that the shapes you cut actually really match. And for any small problems we always have a filling paste.

I think, the ceiling turned out very well as you will see on next pages with real photos.