1. Find the corresponding paragraph that relates to the aircraft you want to fly. In the case of the 737-800, the choices are BGN , BGA (Australia) BGAM (Mail) & BGAS. If you don't wish to download the full set, delete all other BG referenced paragraphs that you haven't dowloaded. DO-NOT delete the original which will read [fltsim.0]
2. Change the numbers that are shown in blue so that any remaining aircraft are in consecutive order (fltsim.1 fltsim.2 etc...)
The fltsim numbers refer to which order the aircraft has been installed. You only ever need to change this when adding another aircraft. In the case of the 737-800, if you download all aircraft, you shouldnt need to change this. If you don't download all the planes then you only need to establish a consecutive order.
Title refers to the main title of the aircraft. I have added initials BGN in the sample to show its BGN model.
texture refers to the folder the textures are in. Uppercase letters are usually used in the config file , lowercase for the folder name ie texture.bgn. Often you may install textures that don't show up. You may need to check that the folder is lowercase and the reference is in uppercase.
atc references are only for offline flying with atc. Set your atc to 99-100% to see all aircraft. You need to have the soundfile for the airline for fs to show it up. You also need to add it in airlines.config using notepad (found in aircraft). I use editvoicepack available from most download sites for free. If saved in edvp then you need to enter the airline in uppercase to the config.
UI files are where things show up in the simulator.
ui_manufacturer is where your aircraft make will show up, ie Boeing, Beechcraft, Lockheed etc... * make sure that you write it exactly as the sim shows it. If you write `BOEING' then it won't show up in the `Boeing ' folder or the `boeing' folder and you may just end up with 3 folders for one builder.
ui type is the model of the plane ie 737-800, C-17, Volpar. What ever you want to find the plane as, that is what you write. If you want to write a plane as a fluffy duck then that's what you put.
ui_variation is where you put the airline/division or whatever. Its where you've found your plane by make & model and now you're looking for an airliner that suits you ie `Bluegrass', `QANTAS' or in the case of some aircraft, `dirty version'. This refers to the textures themself. This is the main ui to change when adding textures. Generally speaking, you should not need to change any other ui if youre using the same make/model.