As this is an English forum, I won't answer your question in German, nevertheless I hope you will understand me!
I will keep this very simple, so you will be forced to make use of your own creativity
At first you will need to have some things:
+ access to the 3D model (very important
)
+ a 3D program to view and or export the mesh's mapping
+ painting program capable of import and export DDS textures or a paint program and a DDS converting software
+ knowledge of using the painting program, especially: paths, gradients, importing textures/photos and adapting them to your needs
+ for more advanced things like shadings etc. you would need the knowledge of techniques to export rendered shadings from a 3D program (alternatively you could do shadings just by "painting" with gradients etc.)
So the single steps would be something like this:
+ importing the model into your 3D program
+ visualize the UV-mapping and take screenshots of it (you can then import them into a painting program and attach them to each other so you get the full UV-mapping) or even better: export the UV-mapping to an image (e.g. Blender and 3DStudioMax can do this job)
+ create a new image with your painting program in the same size of the texture mapped to the 3D model (e.g. 1024x1024, 2048x2048, 2048x1024....) just to keep things simple
+ import the UV-map and remove unneeded parts of it (e.g. background)
+ import the shading (if you created and exported it before!) and set the layer to "multiply" and maybe adjust the layer's transparency to your needs
+ start painting details: use paths for bodylines and other details which can be painted this way, use gradients for additional shadings etc., use textures for headlights, taillights, screws, plastic bodyparts etc. etc.
+ save your template in a format which is capable of layers (to have a "common" template)
+ export your template as DDS to see your results on the model (importing the "finished" texture in your 3D program for example)
other points to put on your agenda could be:
+ use your template to generate a specularmap and a bumpmap
+ make use of the alpha chanel (same as with the specularmap, but integrate the "specular" into the final skin's alpha channel)
hopefully this can give you some kind of impression of what to think of and what steps you will have to do. maybe this is just a very generic blueprint to simply start somewhere
I would suggest to simply start making a very simple template...later you will be able to improve your work step by step
