install in a virtual NS6 nethserver-mock then you will be ready
after in a folder of your choice
git clone https://github.com/NethServer/nethserver-openssh.git
you will retrieve the source, ready to be read
when you have modified something do
git commit -m 'my message of commit'
and build your rpm
make-rpms
read the full example, all answers to your questions are in /usr/share/nethesis/NethServer (of the git folder you have just cloned) and Creating web UI module — NethServer 7 documentation
some module examples : GitHub - NethServer/nethserver-ui-examples: NethServer UI examples
but you have other clues more developer oriented
http://dev.nethserver.org/nethgui/Documentation/Api/index.html