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
i've got 1 fan sukking the air out of my system. there's no room voor a front fan... but would i need to have a fan blowing air in to my system. if so i would rather not use the top 1 for because at the LAN its quite dusty