Some find it easier to organize local code, when the structure of code directories matches the structure in a github group. For example, if you belong to a github group called Vogons and you have a repo called Jeltz, the structure in your github will likely look like Vogons/Jeltz. To reflect, one creates a directory on their local machine at something like ~/code/Vogons/Jeltz.
Suppose one has another repo in the Vogons group called Kwaltz, and one wants to make Kwaltz a submodule of Jeltz. To reflect this organiziation of code on github locally, one wants Kwaltz repo to reside at ~/code/Vogons/Jeltz/Kwaltz with a symlink at ~/code/Vogons/Kwaltz.
Fist, assume one already has Jeltz and Kwaltz at their respective local directories, ~/code/Vogons/Jeltz and ~/code/Vogons/Jeltz/Kwaltz.