INSTALLING NEW SKINS.
1) First off you need to download your skin, I'm assuming you've already done this. So go to next step.
2) Locate your Documents on your C:\ drive.
3) Now browse to the following destination.. Windows versions may vary.
Windows XP :-
C:\Documents and Settings\YOURWINDOWSNAME\My Documents\TrackMania\Skins\Vehicles\CarCommon
Windows Vista :-
4) Note: If you're missing the folder "CarCommon", then create the folder with the exact name I've typed, but do not include the quotation marks.
5) Place your skin into the directory, and close.
6) Open up your game, and browse to your profile. Select Vehicles and select your newly installed skin.
--- If you have any questions concerning this tutorial, please don't hesitate to ask.