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1. Unzip into your /wp-content/plugins/ directory. If you're uploading it make sure to upload the top-level folder. Don't just upload all the php files and put them in "/wp-content/plugins/".
2. Activate the plugin through the plugins menu in WordPress
3. Open comments.php and search for "<textarea". Check the id of the HTML tag. It has to be "comment" (id="comment"). This is WordPress standard.
4. Paste following PHP code in the line before "<textarea ...": <?php if (function_exists('quicktagzmilies')) { quicktagzmilies(); } ?>
5. Save and upload comment.php. The quicktagbar should now appear in your comment area
6. Go to Quicktagzmilies options in your admin panel (Settings » Quicktagzmilies) to customize the plugin

Please check Quicktagzmilies page for further information