To be quite honest I think if you have installed a skylight (especially if it has a powered fan) you do not really need a separate extraction fan for cooking,
That said if you do need to install one this is what I installed on my first van.
I cut a hole in the roof for the extraction fan – it has to sit within the runnels else the rubber seal will not work.
The extractor fan simply has to fit in to the hole and be screwed to the vent part. I had to find some longer screws for it to reach. The added insulation and cladding meant the screws that came with the unit were not quite long enough.
This was the mushroom vent I used:
It cost about £30 / $40
It is one of the few 12V vents I could find online – and to be honest it is not the greatest product. It is quite loud but it does work effectively.