This is a detailed tutorial on how to install a plugin. For the purposes of this guide, I'll be using the Donate Sidebar plugin as an example. But these instructions can be used for other plugins. Obviously replacing the references to Donate Sidebar to the other plugins name.
Download the plugin and unzip it's contents into a folder on your desktop if needed. Most plugins will have a basic folder structure like this. Where the plugins xml file is usually in the root folder along with the readme or instructions files.
In your browser open up your Admin CP and navigate to the System Tab and then Plugins menu. At the top of this page you will see the 'Install New Plugin' button, click it and find the plugins xml file you just unzipped onto your desktop. Once your ready, click the Install button to start the import. The plugin installer will then run through the installation and you'll see several redirects until it's fully installed.
Step Three (Disable/Move/Settings)
After you've installed it, you'll be able to enable/disable, move and edit any settings the plugin may have. Use the drag and drop icon to reposition a plugin and disable a plugin by toggling the status badge. If a plugin has any settings for you to modify, click the edit icon to modify any settings a plugin has.
Step Four (Widgets)
Plugins now come with the ability to add widgets to your forums. If your plugin does have a widget available, Open your forums home page and locate the "Manage Blocks" icon on the center left hand side of your forums. Clicking this will load a side menu with all the available plugin widgets, locate the widget for the plugin your installing and then drag and drop it to what ever widget area position is available..
If the widget requires further configuration, you'll see an "Edit" button appear otherwise the widget will be shown straight away. Click the "Finish Editing" button and your widget has been saved and will display on future page loads.