If you want more than just a skylight and want a fan/vent/skylight combination, I just installed 2 of these this weekend:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
I have never done anything like this and it was easy (much easier than I thought). The emergency exit was 24 x 24 and these vents were 14 x 14, so it required a 27 x 27 piece of sheet metal to cover the gap. I used this caulk for under the sheet metal, along side the sheet metal, and along side the roof vent:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
And I used this putty tape for under the fan/vent:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
And then TEK screws to hold down the new sheet metal and to attach the fan/vent to the new metal piece. Finally primed it, and covered the edges with Butyl tape from Home Depot as a third level of leak protection.