-a software to edit .PSD files : photoshop (shareware) or Gimp (freeware)
-download the .PSD files pack of the 4 F-jets
Then there is a note in the pack that is :
1. In your NSR Stage 1 installation folder (by default its name is "NSR_Stage_1"), create a folder with the name : "Creation\Textures\Skins".
2. Open with Gimp or photoshop the PSD file corresponding to the F-jet you wish to paint.
3. Draw on the "my skin" layer


4. Save your creation as .tga file (32 bits/pixel, uncompressed, with the alpha channel)

5. Delete one of the textures in the folder "NSR_Stage_1\NSR\Resources\Textures\Skins", for example "StuntCarG3_1_03.tex"
6. Put the new tga you've saved at step 4 in the folder created at step 1 "NSR_Stage_1\Creation\Textures\Skins", and give it the same name as the file you've deleted, which means "StuntCarG3_1_03.tga" in our example.
7. Launch the game and go in the F-Jet selection page, and click on "3D view" to see your new skin (you'll have to wait a bit until it's converted).
So you just have to do the best you can and share your creations with us

Have fun !