Hey Jared, good to see you again. I am a huge FSX gamer and it was FSX that pushed me into actual flight lessons. That being said, let me see if I can help you in setting up an ILS approach in FSX.
FSX is a wonderful tool for instrument practice. I am beginning my instrument training and I am moving very fast because of my work on FSX. In FSX you can shoot actual approaches with the actual procedures in actual approach plates. For instance, click this link -> http://204.108.4.16/d-tpp/0904/00246IL35...
The above link is an approach plate for the ILS/LOC RWY 35 approach in Manchester, NH (KMHT). Pending you can read this plate, this gives you all of the information you need to know to shoot the ILS 35 approach in real life and on FSX.
Let me run through the plate for you, just in case it is a little foreign.
First the top portion: The top lists all of the information relating to the airport, procedure you are executing and runway information. The most critical information you need to take from this is the ILS freqency (109.1).
Moving to the middle of the page: This is a top down view of the approach as if you were watching it above the airplane. You'll see here a lot of information. Lets look at this as if you were approaching the airport from the south. If you were approaching this without ATC vectors you would want to fly direct to Lawrence VOR (LWM). 112.5 should be in either one or both of your NAV radio's. This is one of the initial approach fixes to run the ILS procedure. Once over LWM you follow the 290 degree radial outbound to intercept the runway 35 localizer which is 353 degrees inbound . Per the approach plate you will intercept this localizer 10.4 miles from Lawrence. At this point, to make it simple, you would turn inbound to KMHT on the localizer for runway 35. In actuality you sometimes need to execute a procedure turn, but we won't get into that right now. As long as you follow the localizer all the way into the the airport you will be perfectly lined up with runway 35. However, how do you monitor how high or more importantly, how low you should be? The answer is at the bottom of the approach plate.
At the bottom of the plate you will see another diagram. This is the same approach on the middle of the approach plate but from a side view. It is meant to show you what your altitude should look like on approach (if you do not have a glide slope function on your plane as an ILS is meant to also give you altitude information on approach). On this chart you will see that you CAN NOT be lower than 2,000ft at PELAN and may not descend below 2,000ft until you pass POPVE. Once you pass POPVE on the approach you can begin descent from 2,000ft until your minimums which are below the chart. If you are executing the actual ILS into runway 35 you can descend until 465ft MSL. However, you may not go below this altitude. You may ask, how will I know when I cross POPVE or PELAN. Quite simply, when your NAV1 radio is dialed into the LOC 109.1 you need to have your NAV2 dialed into LWM or MHT. If you dial into LWM (112.5) then you will cross PELAN when the NAV2 radial is 290 degrees outbound and will cross POPVE when the NAV2 radial is 311 degrees outbound.
Finally, to the left of the altitude information is a chart of the airport and below that is time information. What this says is that if you are traveling 90kts ground speed it will take you 3 minutes and 28 seconds to reach the runway. If you do not see the runway on or before this time is up, you must go missed and execute the missed approach procedure which is at the top of the page.
I know I just gave you an approach plate crash course. If you have any additional questions, please ask me.
To better answer your question in relation to FSX and programing the information into FSX. It is possible to input this information into the GPS and it is quite easy to execute the approach. However, as a instrument pilot student, I would reccomend using the NAV radios as they it provides an instruction that you will most likely be using in actual training. Don't get me wrong, it is great to use all instruments available in an aircraft and to be trained on all but the NAV radio's are a lot of fun.
You can pull any approach plate from Airnav.com and you can use it on FSX. There are all sorts of approaches that utilize different technology and different instruments. Have fun with it and try new stuff.
Also, you can open an IFR flight plan with the FSX controllers and the can vector you for these approaches.
Lastly, (I promise) if you are interested in flying real world approaches with top notch ATC, I would reccomend joining Boston Virtual ATC which is an online multiplayer server on FSX Online. I belong to this community and it is absolutley amazing. I myself am I certified controller on the server. The server prides itself on acting exactly as actual ATC/pilots do. We have weekly events that range from regional hops (150nm flights), to domestic journey's (300nm flights), to GA fly-in's and landing competitions. It is a great place to share information and have fun. I would be more than happy to give you the password sometime to see if you like it. The link is in my sources.
Till next time....Happy flying!
EDIT: To answer your secondary question, yes you need to be in line with the "extended centerline". In order to execute an ILS localizer you must intercept the signal which extends approximately 15-20 miles out from the runway. In my example above you should fly a heading no greater than 30 degrees from the runway heading which would be 023 degrees or 323 degrees to ensure a smooth catch of the localizer (runway 35 in KMHT is a heading of 353). You must also be below the glide slope to catch the glide slope. If you are above the glide slope you will be able to track the localizer in bound but it will not control your vertical pitch.
Do the following as a test:
Use a Cessna 172
Dial in an ILS freqency of a runway of your choice in your NAV1. Depart from that runway and fly a left hand traffic pattern. On downwind you will notice that the "needles come alive". Upon reaching the end of the runway your localizer needle will be fully deflected left and your glide slope needle will be all the way to the bottom of the guage. As you fly further out the glide slope needle will begin to climb. Fly out about 10 miles from the approach end of the runway and once the glide slope needle is to the top of the guage turn back towards the runway. Once you do this the localizer needled will be fully deflected right but as you get closer to the extended center line the needle will come alive. Your goal is to keep the needled perfectly centered on the guage. Once you are tracking the localizer inbound your glide slope needle will begin to fall. Again, keep that needle in the center of guage. If you do this all the way to the runway, you will be perfectly lined up and at a perfect altitude to land. This same procedure can be done with any plane but I would use the 172 to keep things slow so you can learn.