First off, I’d recommend that you do one on the side, and one on the top, as a blow hole. This way you get good flow instead of two fans trying to pull air form the same place, creating a possible "dead zone" of air.
To physically do it, get your self a key hole saw (looks like a reg drill bit with a cup mounted around it) drill you hole, mount the fan AND A FAN GUARD!!! Use self tapping metal screws to hold the fan and guard in place