1st step :
Find some skins for trackmania. The most famous site who we can find some car skins is TM-carPark
there you'll have a .zip do not uncompress it! place it on a place where you can fond it easily.
2nd step:
Open the ( keyboard shortcut : windows + e).
and follow the following PATH c:/Program Files/TmNationsForever/. There you can see the games files.
continue to follow this path : .../Game Data/skins/vehicles/stadiumCar and place your .Zip here.
3rd step:
run the game. After go into profile, clik on vehicle and shoes your new skin.
you must have a think like this
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/!\it's only a car skin it dont change anything in the size or the car's performance /!\