1. Generate your custom like button and get script.
http://developers.facebook.com/docs/reference/plugins/like/
2. Modify your wordpress theme file.
e.g. /wp-content/themes/theme-name/content-single.php
Find “</header>” and insert FB script before it.
<iframe src=”//www.facebook.com/plugins/like.php?href=<?php the_permalink(); ?>&send=false&layout=standard&width=450&show_faces=false&action=like&colorscheme=dark&font=verdana&height=35&appId=233086249908″ scrolling=”no” frameborder=”0″ style=”border:none; overflow:hidden; width:450px; height:35px;” allowTransparency=”true”></iframe>
Replace href value to “<?php the_permalink(); ?>”.
Note: Test on WordPress 3.4.2.