Download the plugin you want to install.
Connect to your FTP server by following the guide: How To Edit Files Using FileZilla FTP
Copy your plugin files with these extensions as follows:*.smx
to the folder /csgo/addons/sourcemod/plugins*.cfg
to the folder /csgo/addons/sourcemod/configs*.sp
to the folder /csgo/addons/sourcemod/scripting
For all other files, follow the instructions on the page you downloaded the plugin from.