Take your fan, set it in the position you want it on top of your router, and tape it down.
Hook up your Dremel drilling tool; make your pilot holes for the screws.
Remove the tape and fan, attach your cutting tool to the Dremel and start cutting the pre-punched area on the top of the router.
Here I attached a switch on the large fan so it can be turned off if it gets too loud. I made a hole behind the fan for easy/ concealed access. Super glue on the bottom and a little Elmer’s glue with ink from a Crayola marker mixed in on the top, and were good to go J