Renovation: add/cut part of a slab

[alemanda]

How do you add part of slab or create an opening on a slab and show them correctly in the "new" status?
It is easy with doors/windows but I'm not finding an elegant solution with slabs
In other words, is there a simple way to handle openings in the slabs assigning a new or to-be-demolished status?
Any help is appreciated
Thanks

[alemanda]

... I'm just using the skylight with a flat roof ...
any ideas on how to do with the slabs?