1) You will first want to backup your server files on to your computer if there is anything you do not want to lose. After you have backed up the files to your computer, delete all files and directories in your FTP.
2) Create a new folder on the desktop of your computer with any name you'd like. Go to files.minecraftforge.net and select the version you need, then click the installer icon and download the installer. Once it has downloaded, move the "forge...-installer.jar" from the download folder into the folder you created on your desktop. After the move is completed, open the folder and right click the "forge...-installer.jar". Select "Open with > Java(TM) Platform SE binary". Select the option to "Install Server", then change the installation destination to the folder you created on your desktop. You will get a message in red saying that one of the files already exists, ignore that and hit ok. Once the process is done, you will get a pop-up telling you it has finished, click “OK”. You can now go back to the install folder and delete the "forge...-installer.jar" as you will not be needing it anymore.
3) Next go to the install folder, inside should be 3 items. "forge...-universal.jar", "minecraft_server...jar" and a folder named "libraries". You will need to rename the "forge...-universal.jar" to "forge.jar".
4) Now head to your Multicraft panel and find your server. Once there look below the section that says "JAR File" and has a textbox next to it, you need to click the drop down arrow and set it to "Forge".
5) For this step, you’ll need to use a FTP client to continue. We recommend FileZilla, which you can download at http://filezilla-project.org/download.php. Log in to your server using the FTP details located on the “FTP File Access” page in Multicraft. You can find this by choosing “Files” and then “FTP File Access”. Once logged in, create a directory in your FTP named "jar". Upload the “libraries” folder, the “forge.jar” file, and the “minecraft_server...jar” file from your folder on your desktop.
6) Once you have finished uploading those 3 items, head back to your server on Multicraft and press the “Accept EULA” button about halfway down the page. Next, press the "Start" button up at the top to start your server. You can look in the console to see what is going on, but shortly your server should be up and running. Now that your server has successfully loaded, all the necessary files and directories will have been created in your FTP. You now have successfully installed Forge.
7) Now for the fun part! It’s time to install your mods. Just as you did in step 5, you will need to move any mods you have downloaded from your computer to your FTP directory named "mods" using your FTP program (probably FileZilla). *NOTE* For those of you new to creating your own modded server, you must also have all mods installed on your server inside of your ".../AppData/Roaming/.minecraft/mods" folder as well to maintain compatibility. Also make sure not to download any client-only mods on your server or your server will NOT run.
8) Once all of your mods are installed in your "mods" folder in FTP you are now ready to head back to Multicraft to restart your server one last time. The startup process may take some time, depending on how many mods you have added, but in a few short moments, your server will be up and running with all of your mods installed!
If you need any assistance with getting your mods or modpack installed, consider purchasing our Modpack Setup and Support addon and we’ll be happy to assist you. You can purchase this addon and others at https://ggservers.com/billing/cart.php?gid=addons.