Unzip the compressed file and upload the flexi-pages-widget.php file (or flexi-pages-widget directory) to the /wp-content/plugins/ directory
Activate the plugin 'Flexi Pages' through the 'Plugins' menu in WordPress admin
Go to WP admin -> Appearance -> Widgets, add the 'Flexi Pages' widget into the sidebar and choose your options. Multiple instances of the widget can be added to the sidebar.