Follow these steps to use a custom jar or modpack:
- Select "custom.jar" from the server jar menu, click "save" at the bottom of the page.
- Upload your new jar to the "jar" folder via FTP. (Make sure to name the jar "custom.jar")
- Upload your modpack files to the "base" or "/" directory of your server via FTP.
- Start your server and watch console / attempt to log in to verify the server is running correctly.