Before anything, make sure the texture you are downloading is for the exact aircraft file you wish to add it to. If, for example, you have a texture that is designed for the default 747, it can only be installed in the default 747, and not likely ano independently developed 747 add-on.
1) Download a "texture" file just like any other download, to a temp download folder that you create, if you do not already have one.
2) Unzip the dowload to a temp location, such as the download folder, or some unzip to the desktop. To unzip, right mouse click the zipped folder, and select "extract". and follow any prompts.
3) After un-zipping, you should find a new file containing a texture folder and there should be a "read-me" file, at the very least.
4) Copy or cut the texture file into the aircraft's folder.
5) Open the "readme" file. There should be an entry that looks something like this:
[fltsim.XX]
title=Albatross_DIII
sim=Albatross
model=
panel=
sound=
texture=
kb_checklists=
prop_anim_ratio=0.35
ui_manufacturer=Albatros (OAW) Collection
ui_type=D.III
ui_variation=Jasta Unknown - Pilot Unknown - 1671/17
atc_id_color=0x00000000
atc_id=D3
visual_damage=0
description=The Albatros D.III is one of the classic fighters of WWI, despite being plagues by wing failures under extreme flying conditions. Manufactured initially by Albatros, production was moved in Spring 1917 to the OAW subsidiary, to allow Albatros to concentrate on D.V production. The most noticeable difference between the aircraft was the shape of the rudder, which was larger on OAW manufactured aircarft, as depicted in this model. Powered by a 6 cylinder 160hp Mercedes water cooled engine.
atc_heavy=0
atc_id_font=Verdana,-11,1,600,0
6) in your aircraft folder, there is a file that shows up as "aircraft". Copy and paste that to your desktop and opwn it. At the top of the page, you sill see a similar entry, and if the aircraft already has more textures than one, you sill see several entries.
7) scrol down to the last [flightsim.ZZ] (where ZZ is the last sim. If there is only on sim, it will be [flightsim.0] If there are three variations (paints), it will be [flightsim.2] )
go back to the readme file and cut or copy the [flightsim.XX] entry.
9) Go back to the aircraft.cfg on your desktop and paste the [flightsim.XX] right below the last sim entry that is already there.
10) If there was only one sim there before ([flightsim.0]), change the "XX" in the part you just pasted in to [flightsim.0] or whatever the next sim number would be if there are more than just one.
11) Go back to the aircraft folder in the game where you copied the aricraft file from, and rename it "ORIGAircraft" This is to make it a back-up file.
12) Back at the desktop aricraft file, save the changes to the desktop file, then cut or copy and paste it back into the games specific aircraft folder.
13) (Optional) Close folder, lock door, run in circles chanting magical incantations.
14) Start FSX and check to see if the new version is there. Make sure the "show all variations" box is checked at the bottom of the select aircraft screen. If it shows up, you should be good to go.
If not, post the exact texture you are trying to add, and I'll (or any of the others here) will go through it again, step by step.
That should be all there is to it, and it's not near as hard as it looks, especially the optional step 13.
