Thanks to Tim for the excellent explanation, I've reposted it here as a sticky for everyone to see and read.
An ILS approach provides a pilot with two narrow beams of radio waves (well, four actually, but we won't get technical). One provides horizontal guidance over the ground, and one provides vertical guidance above the ground. By combining the two together, you get very precise guidance down to the runway for landing.
For this description of how to fly an ILS approach, I will use the assumption that you are receiving vectors from ATC. The procedure will be a little different if you are not in contact with ATC, and will most likely involve a procedure turn or arc, which definitely make things a little more interesting.
At some point prior to reaching the airport, the controller should have informed you what approach to expect. This will tell you what type of approach to expect (ILS, GPS, VOR, Visual, etc), and what runway to expect (27, 08, etc). As soon as you know what approach the controller is planning on giving you, start getting it set up. The first thing to do is tune in the localizer frequency. If you happen to have a chart for the approach, the localizer frequency will be printed on the chart, usually in a box with a line drawn to the "arrow" representing the localizer course. You can also use AFCAD or the FS map to get the airport information, including frequency for the ILS on the runway you will be using. While it is not necessary, it is generally a good idea to also dial the correct course of the localizer in as well to avoid any confusion and provide better situational awareness.
The controller will bring you down to the correct altitude (typically around 3,000 feet AGL, but it varies), and will usually try to have you intercept the localizer at about a 30 degree angle. If the autopilot is flying (like it should be
), make sure the APP button is engaged. When the localizer needle (the needle or arrow that moves horizontally across your display) begins to move, the autopilot will disengage the heading mode and begin a turn to intercept the localizer. "Intercept" means to center the needle and keep
it centered (if you are flying manually, your job does not end when the needle is centered). On VATSIM I am frequently asked to "report fully established on the localizer." This means that once the needle is centered, you want to notify the controller that you are following the guidance of the localizer towards the runway.
As you maintain altitude, the glideslope needle (the needle or arrow that moves vertically across your display) will begin to come down. That indicates that you are beginning to approach the glideslope from below. Never
attempt to capture the glideslope from above. When it centers, the autopilot will disengage the altitude hold mode, and begin following the glideslope down towards the runway. This is typically the point at which the landing gear are extended.
As you descend, aim to have the aircraft fully configured for landing by the time you are 1,000 feet AGL. That way the autopilot already has the aircraft trimmed for you, and when you disengage the autopilot to make the landing, you won't be surprised by a sudden pitch change. In most cases you will disengage at 200 feet, but depending on weather and the accuracy of the ILS, this can be as low as 100 feet, or even allowing the aircraft to land itself.
In cases of bad weather and poor visibility, what category of ILS you are using will determine how low you can go. With most ILS approaches, you cannot go any lower than 200 feet AGL unless you have the runway approach lights in sight, in which case you can descend to 100 feet AGL. If you do not have the runway itself in sight (this could mean you see the actual runway, the runway lights, touchdown zone lights, or threshold lights), you need to go around immediately. Keep in mind that most of the aircraft we fly here are fairly heavy, so stopping a descent and beginning a climb will eat up some more of that altitude between you and the ground.
A CAT II ILS approach will allow you to descend to 100 feet AGL. Once again, you will want to go around immediately if you do not see the runway at that point. There are stories of aircraft going missed and actually having the landing gear briefly touch the runway before the climb begins, so don't waste any time there. And if that does
happen to you, you are already committed to the missed approach, so don't try to convert it into a landing. That kind of thing kills people sooner or later, so don't get any bad habits.
A CAT III ILS approach, is the only type that will allow you to land without actually seeing the runway, and the autopilot [/i]has[/i] to fly the approach all the way down. I know that autoland in FS is usually pretty rough, but there are some aircraft models (usually payware) that do a good job of it. The only way a human pilot can fly a CAT III approach manually is if the aircraft is equipped with a HUD, and the pilot has had specific training. Even then, I believe they need to have the runway in sight by 50 feet AGL. If you go missed at that point, you are almost guaranteed to have the gear touch the runway momentarily.
I know this isn't my best tutorial, but I just woke up and haven't had any coffee yet.
I'm sure there will be further questions, so feel free to ask away. The one thing to remember is that it is not cheating to let the autopilot fly the aircraft. It can fly it a lot more precisely than you can.
<Edited for a minor issue by B. Jones>