Unzip the folder into your wordpress plugins folder.
Activate the plugin through the 'Plugins' menu in WordPress
Add the widget to the chosen sidebar. EG: In Sandbox plus, this would probably be the sidebar 3. (Note many themes do not allow for this possibility, however you can add an additional sidebar to your header see WordPress SteppingIntoTemplates or Anmari's widgetising headers
That’s it - customise the css and background image to taste.