Often when you download a texture there is also a "read me" document where the text you need to copy into your aircraft.cfg file
It should be something like this:
[fltsim.0]
title=Boeing 737-800 Paint1
..................and further on
the next line should begin with (fltsim.1)
A lot of the textures i download has the required text in a "read me"
and then you just change the(fltsim.?) to the next number who is available in your aircraft model.
Example:
If I have a model with last line of texture:(fltsim.12)
I just name the new texture line (fltsim.13) and copy/paste the text who followed the new texture into the aircraft.cfg file.
Sorry if this is more confusing then helpful
Also it might be different from FSX and FS9
If there isn't a "read me" file with the new texture I don't know what you should do.