First, you should have to install the libraries, run composer install.
You have to put your source code in the src/ folder. PSR4 is enable and works, you can check composer.json.
In this repository you could use PHPUnit or PHPSpec, feel free to use whatever you want.
If you use PHPUnit write you test inside tests folder under Tests\ namesapace and if you use PHPSpec is setting up with default configuration and we have added the extension for coverage.
Create a commit for every step that you do. We will be easier for us understand what you are doing and what are you thinking.
We recommend use TDD, if you don't know, don't worry, you could test at the end, and if you don't know how to test, don't worry, it is fine.
We highly recommend use all your knowledgemnt of Object Oriented Software Design and good development practises.
Good luck and enjoy!
WAM Develop Team.