To use Eon you need to have a C++ IDE installed and you also need to setup SFML in the project as well, you can find plenty of tutorials online on how to do this. Clone the Eon repository https://gitlab.elaztek.com/JTKreates/Eon-ge and then copy and paste all of the files from the src/ folder into your project source folder. From there import the files and make your changes. The only files you need to edit would be any of the state classes or the resource holder header file. Editing other files might break the engine and i'd recommend not changing anything unless you are sure you know what you are doing. If you'd like to request a certain feature then just message me and i'll get to it as soon as possible. More detailed documentation will come out further along in development.
Good Luck Developing 👍