This *should* be easy. Follow the direct link supplied here. Then click on the ‘Cancel Account’ button to delete it.
However, at the time of testing, we couldn’t complete the process. If you are experiencing similar problems, leave a comment. For now SL is put on the grey list.
URLS:
http://www.secondlife.com
https://secondlife.com/my/account/cancel.php?lang=en-US