Start by downloading the latest version of FOSSBilling from the official website (https://fossbilling.org/). Follow the provided instructions to install it.
First, download this repository which contains the Rootpanel.php file. After successfully downloading the repository, move the Rootpanel.php file into the [FOSSBilling]/library/Registrar/Adapter
directory.
Within FOSSBilling, go to System -> Domain Registration -> New Domain Registrar and activate the new domain registrar.
Next, head to the "Registrars" tab. Here, you'll need to enter your specific configuration details.
Finally, add a new Top Level Domain (TLD) using your module from the "New Top Level Domain" tab. Make sure to configure all necessary details, such as pricing, within this tab.