Usually a texture DL will give a link the required base file for that texture. The easiest thing to do is to simply DL the required base file. Otherwise, the texture will not work, since textures are sim model specific.
As far as what to do with a texture file, the DL's come with instructions, and are easy as pie:
Extract texture DL.
read "readme", since it will usually come with the needed "fltsim" edit for the aircraft.cfg file.
Place the "texture.whatever" in the specific aircraft folder.
Open the aircraft.cfg and add the copy and paste the "fltsim.x" from the "readme" file into the .cfg under the last 'fltsim.w' entery. Change the "x" the the next fltsim number. If the basic aircraft already has one texture, it will be [fltsim.0], so your add on texture would need to be numbered [fltsim.1]. Save the changes. (If you use Vista, or Win7, and installed FSX in it's default location, copy the aircraft.cfg to your desktop, then do your edit, then save, and copy or cut/paste the .cfg back into the specific aircraft folder.)
See? Nothing to it.
