Assuming you already have a plugin file downloaded on your computer:
1. Locate the file you downloaded. It will likely be in your Downloads folder.
2. Right-click (or control-click) on the file and select Show in Finder.
3. A new window should open showing the contents of your Downloads folder. Find the plugin file you downloaded (it will likely have an .osx or .dmg extension) and double-click it to open it.
4. A new window will open with the plugin installer inside. Simply follow the prompts to install the plugin, then close the installer window when it's finished.
5. The plugin should now be installed and ready to use!