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Extract all files from the ZIP file, making sure to keep the file structure intact. Rename the folder to "wp_bf2s" and then upload it to /wp-content/plugins/.

This should result in the following file structure:

- wp-content
    - plugins
        - wp_bf2s
            | readme.txt
            | screenshot-1.png
            | template.po
            | wp_bf2s.php
            - images
                | close.gif
                | rank_0.gif
                | rank_1.gif
                | rank_2.gif
                [ ... snip ... ]
                | rank_19.gif
                | rank_20.gif
                | rank_21.gif

If you are using the Sidebar Widgets plugin, then you're done.

If you aren't, then you'll need to edit your theme's sidebar.php file. Add the following code where you'd like the output to show up:

<?php if (function_exists('wp_bf2s')) { wp_bf2s(); } ?>