Forge is the most used method of adding mods to a client and a server. It is fairly easy to setup, all you will need is a SFTP client and the mods you wish to install. We do have Forge on our version changer or we can manually add it which is what we will be doing here.

You can NOT use plugins with forge!

First, download the forge installer from here. Make sure to download the version you want to install. We recommend downloading recommended as it is the most stable build.
Once installed, make a folder on your desktop. Click install server, and select the path to the folder you created on your desktop.

Once the files have downloaded (see below). Rename the forge jar to server.jar (or whatever your startup file is).

Run the server.jar, then you should see the files below.

Connect to the server via SFTP, and move all the files from the folder to the server.
After all the files have been transferred to the server, start the server.
Upload the mods you wish to have on your server to the mods folder, then restart your server.