Ok I know there are many people out there who aren't sure how to install aircraft textures onto base models, so I'll take the time to show them all how. It's not as confusing as it may seem.
First, you obviously need to have your model and your texture that you wish to install. Once you have read the Readme file and made sure you have the right model (like for example, a Project Airbus A321-211 with CFM engines), go to the base aircraft (the model you wish to install to) and copy the aircraft.cfg file. Paste it in a folder of your choice- I have one called backups. This means that if something goes wrong, you can just paste it back to it's original place.
Once you have copied it, make another copy and paste it onto your desktop. You don't have to put it here, I just prefer it. Open it with Notepad, and go to the bit that starts with:
[fltsim.1] (or something similar)
And then go to the very last entry starting with [fltsim.whatever number is next].
Open the Readme file of the texture you want to install and copy the bit it says to copy.
It will be another bit starting with [fltsim.xx].
What you need to do now is paste this entry into the aircraft.cfg file you should still have open. Paste it after the very last entry starting with [fltsim.whatever], and where you have [fltsim.xx], you need to replace the 'xx' with the next number in sequence. Like [fltsim.9] or whatever it may be.
Once you have done this, save it. Remember, you must have pasted it onto the desktop for it to save without the 'save as' window opening. This ensures that you save it as a .cfg file, not a .txt file.
Close the aircraft.cfg window, and cut the file from the desktop and paste it back into the base aircraft folder (the one with the model). Then, take the texture folder you want to install (the whole folder, not just the internal files) and copy it into the base aircraft folder.
This should have correctly installed it into FSX, and you should now have your textures working.
NOTE: If you change the name of the texture folder, it won't work. In the aircraft.cfg file, there is a section that says:
texture=texturefoldername (for example)
If you change the folder name, the cfg is looking for a file that does not exist. Therefore, it will not work. Keep the folder name the same at all times!
That's it, your textures should now work. I hope this guide was helpful to you!