A roof window is a good first rooflight for a homeowner to consider, because it sits in the slope rather than on top of it. The covering is cut back around the opening, the frame is fixed to rafters, and flashings are dressed over the tiles or slates so water runs around the window, not behind it.
Inside, the work is a lined opening from the window frame to the ceiling. In a bathroom that lining takes a bit more thought, because it will be wet with steam every morning. It is insulated, finished with a moisture-tolerant board and painted with a bathroom paint, so it does not mark or peel. If the room has a slanted ceiling, the lining is angled to match, and a deeper void means a longer lining.
Controls deserve a thought too. A bathroom roof window that opens must be reachable by someone standing on the floor, or have a pole or an electric operator. The height of the window up the slope decides which. We measure from the basin to the handle position before we agree where the unit sits, so you are not stretching over a bath to reach a catch.
The usual sequence is below. On a simple single window, the roof stage typically wraps up inside a day or two.