How to install Plugins on your Minecraft Server
1) Login to your Panel Account at https://panel.humbleservers.com and click your server's name
2) Make sure you server is either craftbukkit or spigot. If you want to change to spigot or craftbukkit follow this guide: https://billing.humbleservers.com/index.php?rp=/knowledgebase/7/How-to-Change-your-Minecraft-Server-Version.html
3) Download the plugin's .jar from the plugins page, major plugin pages are: https://www.spigotmc.org/resources/categories/bukkit.4/ and https://dev.bukkit.org/bukkit-plugins
4) Once you have the .jar downloaded, locate to the plugins folder in your file management tab
5) Click the black upload button located on the top right, and select your plugin .jar
6) Go back to your console page and click the restart button, and that's it!