> I just wonder, that neither the official Git manual, nor the kernel
> hackers guide to git describe the best practice to fork a project.
Well, in Git, technically, any clone is a fork. The difference between
a contributor's repository and a forker's is social, not technical (I
usually define "fork" as a branch where one of the projects has no
intention of merging the other's changes).
So, just clone and publish (push) your changes, and you're done.
Or, ask a more precise question, I may have missed something ;-).