Get the fan for the blowhole and a holesaw that matches the size of the fan you'll be putting in. Place the fan where you want the blowhole to be, then, using a dry erase marker (because it comes right off with a little bit of water), trace the fan on the case. Be sure to mark the screw holes too. Take the fan off the case, then set it back down and make sure the outline is correct. Do this a few times, because if your outline of the fan is wrong, then you'll just end up with a big ugly hole in the side of your case. Now you get to go drill out the 4 screw holes. Drill one, then stop. Set the fan on the case and align it to the screw hole you drilled. Make sure that the screw holes on the fan align with the ones you drew on the case. Drill the next screw hole, and check the alignment again. Continue doing this until you have all 4 screw holes drilled. Now, get out your holesaw. Make sure to use a corded drill (not cordless), because they are a lot more powerful. Put your holesaw on the drill, then drill the hole. If you want to, you can then file down the jagged edges of the cut with whatever suits you best, or you can just leave them jagged. Screw your fan and a grill in, and boink, you've got a blowhole.