There currently isn't a tutorial I'm afraid - the best way to create a mod is either using csm to create a mod from a normal gp4 installation and then editing it, or if you want to create it from scratch then try looking through other mods to work out how the file system works etc.
If you need help, there's the csm forums over at [
grandprixx.com]
Hope that helps
Rob
Batracer - the online racing manager game - join here
[
batracer.com]