An attic skylight shaft is constructed between the skylight curb at the roof, and opening on the ceiling.
It can be beveled outward toward the bottom for maximum light spread and presentation.
When cutting rafters and ceiling joists, it's important to note that one is altering the structure of the roof which is designed to support a given weight like roof tiles, comp shingles, etc...
Be cautious when hiring a contractor to perform this task. The appropriate license is general and roofing.
This is also a good example of a project that should be permitted.