*For this you will need Jar Access. If you do not have jar access. You can change it under "advanced settings" in your multicraft control panel.
1. Stop your server
2. Open your FTP client (like filezilla) and connect to your server
3. Using FTP, upload the custom jar and other relevant files to the main directory.
4. Make sure the name of your custom jar file is custom.jar
5. On the main panel page, select custom jar file use this, from the dropdown menu.
6. Click the save button
7. Start the server