It depends on what you will be working on. If you want to improve the UI for the sales page for example, you only need to fork the sales module and change in that part.
Once you fork it, you can install it with "pip install -e git+
https://github.com/username/coinbox-mod-sales", and it will be in your virtual env, you can work from there.
If you want to work on everything in coinbox, you will then have to fork every module, and you can run then "make develop" with the github URL of your forked core repository.