The steps are as follows:
Step 2: Install Rocket Mod following this guide
Step 3: Download your plugins from the Rocket Mod website. Be sure to unzip the folder
Step 4: Upload the unzipped files to the folder Servers\UnturnedServerByHH\Rocket\Plugins\ using the File Manager or FTP. Help with setting up FTP can be found here.
Step 5: Setup the plugin following instructions given on the plugins page. Most plugins will have permissions that need to be set up before they can be used.
Step 6: Start your server