Welcome to the FAIB forum if you are totally new to installing flight plans here is a brief overview.
What do you need?
1. Models and paints. You can get this from here or sites like these: Avsim.com, Flightsim.com, juergenbaumbusch.de, FlyingCarpet75.com, jcai.dk etc.
Check here for the latest info from the painters themselves.
2. Flight plans. My favorite flight plan maker is AIG(Alpha India Group) but there are many others and some even release under their own name, best bet is to check
the FS2004 flight plans categories in Avsim or Flightsim. note: FS2004 flight plans would work fine in FSX if compiled correctly.
3. A flight plan compiler which turn all the text files you get from a flight plan zip into a BGL you can place in FS and have traffic. I recommend AIFP V2 available on Avsim.
4. Time, like any hobby needs.
What are the txt files I get in the archive of a flight plan?
1. The Airport*.txt file which tells the compiler where the airports in the flight plan are in the world and is used by FS to determine which traffic to show where.
2. The FlightPlan_*.txt file which contain the actual schedules, before each schedule there is a code in the form AC# which tells the compiler what aircraft to associate to that
part of the flight plan from the next file.
3. The Aircraft_*.txt file simply creates an association between AC# codes to actual "aircraft names" in FS, these names are defined as "title" in the various [fltsim.#] section in
every aircraft.cfg file and must be unique or else FS will complain. using the associations in this file the compiler can tell FS which repaint to associate with which flight plan.
4. Readme.txt, one of the problems of this hobby is that people always skip reading the readme file, I suggest you don't, it might contain important info.
5. Repaints.txt(not mandatory) should contain a list of download links and names that should help you install all the repaints needed by the flight plan.
The trouble starts where there are dead links or repaints that were not repainted yet or even totally missing models like the CRJ-1000 or 747-800.
6. Setup.cfg(not mandatory) This file is created by the now defunct MRAI installer and contains some data that should help that program install the needed paints.
7. AIFP.cfg(not mandatory) This file if properly created can help you install entire airlines with the simple click of button inside AIFP V2, it will even download all the paints for you.
My first suggestion is to visit the excellent AIG forums here and specifically this thread where you can learn some basic info about installing AI and using AIFP V2.
For me if the flight plans are not by AIG or don't have an AIFP.cfg file in them the process goes generally like this, having an AIFP.cfg file in the flight plans can greatly ease the installation of AI as AIFP V2 does most of the work including downloading and installing the paints for you.
So what do I do if there is no AIFP.cfg?
1. I decided which airline I want to install.
2. I search for the latest flight plans on the various sites and once I download the file I read the readme.txt for important info about the flight plan and also which repaints and AI models are needed for the flight plans(sometimes in the repaints.txt file).
3. I download all the various repaints and if needed models.
4. This usually take the longest, installing all the repaints into FSX. You need to edit the various aircraft.cfg files and copy the textures into the correct folders etc.
If you don't know much about the aircraft.cfg file here is a nice thread in the AIG forum.
The most important thing in this stage is that the aircraft "title" in the sections be exactly like in the Aircraft_*.txt file or FS wont find your repaints.
5. after the paints have been installed I use AIFP V2 to compile the flight plans, in the process AIFP V2 will also scan your aircraft directories and notify you if missed any paint or
if it cant fine one of the repaint titles.
6. copy the traffic BGL into the correct folder which is FS_ROOT\Scenery\World\Scenery\ and off to check the traffic inside the simulator.
This might sound very daunting and it is at first but you should be optimistic, After a few flight plans you should begin to grasp a better understanding of what you are doing
and it should become easier, also I am working on an application that should help specifically with the installing repaints part so no need to edit aircraft.cfg files.
As a final word I really recommend you register on the AIG forum just one tip of advice, if you search and cant find what you are looking for please provide as much information
as you can about the issues you are having with installing traffic if you decide to post a question.