Cutting the hole in the dome window
Draw on the ceiling the place where you want to position the dome. Drill at the four corners to prepare the screwing. Be careful not to damage any pipes present in the surface and do not forget to support the ceiling with stays to prevent the sectioned supporting beams from collapsing. Once on the roof, connect the four drill points by drawing a line; then cut through the roof with the circular saw following the marking. Extract the piece with a crowbar. The beams around the hole are now visible. Cut them with the hand saw, plan to position the cutout two or three centimeters in front of the hole.
To reinforce the cavity, saw two beams of dimension equal to the distance between two intact bearing beams and fix them at each end with nails, equidistant from the main beams. Solidify everything by positioning two other beams. The whole constitutes a wooden frame identical to the dimensions of the dome. Attach the top of the dome to the frame. If it does not rise enough above the roof surface, do not hesitate to create a wooden formwork to complete the structure.
The waterproofing joint that you are going to spread should unify the roof covering and the dome; you can use either rubber or tar strips, PVC or aluminum. Now place the dome and screw it on the rim. There will be covers to hide the screws to prevent burglars from removing the window from the outside. Operate the dome to verify correct installation. Make interior finishes with plasterboard.