If I understand well your question, the answer is by using the paint editor: http://wiki.maniaplanet.com/en/Paint_editor_%28Stadium%29

thanks, but if i prefer use photoshop, how can make modesl 2D on Photoshop?

Haven't done this myself but it should work I believe, Do what Alinoa said first, just paint it blank white or gray, perhaps create grid lines or area markers for yourself, save it. Then look in the folder which contains the car skin you made. (Just search *.zip in documents) open this file and inside you will find .DDS files. Extract those. You will need this plugin For photoshop from Nvidia https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop that will let you open the .dds and you can then paint on the 2D texture.
Once you are done, place your new files in the .zip and overwrite the old ones.

You can also use that tool to paint on other .dds texture files the game has such as diff norm and spec

Thanks!

引用自 :)：

Haven't done this myself but it should work I believe, Do what Alinoa said first, just paint it blank white or gray, perhaps create grid lines or area markers for yourself, save it. Then look in the folder which contains the car skin you made. (Just search *.zip in documents) open this file and inside you will find .DDS files. Extract those. You will need this plugin For photoshop from Nvidia https://developer.nvidia.com/nvidia-texture-tools-adobe-photoshop that will let you open the .dds and you can then paint on the 2D texture.
Once you are done, place your new files in the .zip and overwrite the old ones.

You can also use that tool to paint on other .dds texture files the game has such as diff norm and spec