First, I'll start with the parts I bought. I started by ordering a subwoofer vent from Crutchfield. I have found that the 4 inch vent (~$6) works well for matching up to a 120mm fan, while a 3 inch vent (~$5) matches up well to a 80mm fan. I haven't done this for a 92mm fan yet, but I think I'd rather use the 3" than the 4" if I did. When the vent arrived, I bought a 24"x18" plexiglass sheet from Lowe's (~$7). That's it. Total cost: ~$12.
Next, I'll list the tools I used. I used a dremel for most of the work. I'm a big fan of the continuously variable speed models, but any dremel should do. The two dremel bits I used were a cut-off wheel and a round grinder bit. I also used a hand drill. The drilling is really light, and even a crappy drill should do just fine. Also, it helps to have a full set of small drill bits so you can be sure to have the sizes you need. Besides that, I also used a ruler (any straight edge), a stanley knife (razer blade, anything sharp really) and a marker. That's all.
Part 1: Make A New Window
(If you want to cut your existing window and not make a new one, you can skip this part.)
First, it's important to note the the plexiglass sheet should come with a protective film on both sides. You should DEFINITELY leave this on so that the plexi doesn't get scratched or scuffed while you work with it.
I started by disassembling the case's existing window. (I think it's pretty safe to assume most people's cases have windows. If yours doesn't, you can either just cut a round vent hole right into the side of your case, or you can cut the side of your case to add a window.) As you can see, the existing window on this case is already cut, but for the wrong size hole, and in the wrong location. It's recently recieved a different motherboard and proc, and so it needs a new (better fitting) vent. The existing hole doesn't change anything in the steps of the guide. I simply removed the plastic rivets that held the window in to remove the old plexiglass. Your case may have screws or nuts and bolts instead of plastic rivets. Just remove the window however necessary.
Next, I laid the old window on the new sheet of plexiglass and traced the shape with a marker. I also made sure to mark where the mounting holes were to make my life easier later. If you forget to do this (or can't for some reason), you can simply mark through the actual holes later when you test fit the window.
Next, I cut the window out from the full sheet of plexi. I started by finding the two long straight cuts. Instead of cutting these with the dremel, you can use a ruler and a sharp knife or "stanely blade" to etch a straight line where you want the cut. Do this on both sides, then just snap the sheet in half on your etch line by hanging it over a table edge on the line and pushing down on the part hanging over. It's much simpler and you'll get a more straight edge. I did this twice to get the general shape, and then used the dremel cut-off wheel to trim the more intricate parts out. The important thing to remember is that these cut edges will hide behind the metal of the case side, so they don't need to be perfect, and you don't have to worry if your marker lines remain. In fact, I didn't even round the corners like they were before, because they didn't interfere and you can't see them anyways.
Next, you need to drill the mounting holes on your new window. Now may be a good time to test fit the new window and make sure your hole markings line up. Start by finding the right size drill bit to fit your case's mounting holes. If you want to be really careful, you can start with a smaller bit first, then use the right (bigger) sized one. It's a bit easier to line up your holes with a smaller bit, and they tend to wiggle out of place less when you're starting the hole. Then once you've got the small hole in place, just drill it again with the right sized bit. The trick is to push VERY lightly on the drill when you do this, as it doesn't take much downward force to crack the plexi. Just set the drill bit in place and spin it up. You'll barely need to push down at all. It'll take longer this way, but you'll be much less likely to crack it and have to start over. Also, bracing the back of the plexi helps a lot. If you can set a piece of flat scrap wood under it, that works very well. Then you can just drill right through into the wood and it will prevent bending and cracking. I used a round trashcan. (Same idea )
There you go, the holes are drilled and you can test mount the window now.
Part 2: Adding The Vent
The first step here is to mark the round hole for the vent to fit through. It's important to make sure it lines up with the CPU fan (obviously). To do this, mount the window in your case side (you only need to put in a few bolts/rivets for this), and then reassemble the case with the new window in place. Then, place the vent on top of the window. Line it up with the fan by looking straight down at it through the window. Then mark the position with a marker. It's ok to mark the outside of the vent because it's actually a bit wider at the base (by the lip). Doing so won't result in the hole being too big.
Now, you need to cut the circle out of the window. Start by making rough straight cuts inside the circle with the dremel and cut-off wheel. This will result in an "octogonal" hole that's too small, but it's only the beginning.
Now switch to the rounded grinder bit on the dremel and slowly smooth the circle out until it's circular (at least roughly). Again, this does not need to be perfect. The nice thing about the subwoofer vents is that they have a quarter inch or so of "lip" to them, so as long as the cut is at least close, it will be hidden by the vent's lip. Once the hole is circular, test fit the vent. If it won't fit, slowly expand the circle little by little until it's just barely big enough. A snug fit makes everything easier.
Now, chances are that the vent is too long and will hit the CPU fan when it's in place. That's why you need to trim it. Start by removing the vent from the window and reinstalling the window and case side (again, just a few bolts/rivets will do for this). Now drop the vent through its hole and measure how far it sticks out. That's the length you need to trim. Mark that length on several points around the end of the vent, then connect them to make the circle you need to cut. Use the dremel and cut-off wheel to do this. If you're picky, you can use the grinder bit again to smooth the bottom of the cut, but it's not a big deal as it won't be that visible when it's all done.
Now just reinstall the vent. That's it, everthing should be cut and in place. Your vent should be fully functional at this point.
Don't forget to remove the protective film from the window once you're done cutting and drilling
Also, if your vent does not fit tight enough for your liking, you can glue it in place from the backside of the window. I use clear Tester's model glue for this, since it's made for plastic-to-plastic applications.
That's all. I hope this is helpful or at least informative