Upload the plugin folder to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
For 0.25+, make sure the folder '/wp-content/' is dir createable and writable, as the cache folder will be '/wp-content/widget-cache'; For the older version, make sure the subfolder 'cache' of this plugin, usually folder '/wp-content/plugins/wp-widget-cache/cache' is writable
Configure the plugin through Admin -> Design -> Widgets(see the Screenshots)