If you want to contribute back to GeoNetwork you create a Github account, fork the GeoNetwork repository and work on your fork. This is a huge benefit because you can push your changes to your repository as much as you want and when a feature is complete you can make a 'Pull Request'. Pull requests are the recommended method of contributing back to GeoNetwork because Github has code review tools and merges are much easier than trying to apply a patch attached to a ticket.
The GeoNetwork Repository is at: https://github.com/geonetwork/core-geonetwork.
Follow the instructions on the Github website to get started (make accounts, how to fork etc...)
If you cloned the GeoNetwork Repository earlier, you set you can now set your fork up as a remote and begin to work.
Rename the GeoNetwork repository as upstream:
git remote rename origin upstream
Add your fork as origin (the URL provided by GitHub CLONE or DOWNLOAD button):
git remote add origin https://github.com/USERNAME/core-geonetwork.git
List remotes showing origin and upstream:
git remote -v
To checkout a branch from upstream::
git checkout -t upstream/3.6.x
GeoNetwork uses feature branches for development, and a pull-request workflow for review: