*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. Download the forge installer server (it is a jar file) to your desktop
3. Double click the jar file. This will create 2 jar files plus a directory called libraries
4. Rename the long jar file to custom.jar
5. Open your FTP client (like filezilla) and connect to your server
6. Using FTP, upload the custom.jar and other jar and the libraries folder to the main directory
7. On the main panel page, select custom jar file use this, from the dropdown menu.
8. Click the save button
9. Start the server