I was sent this today I was asking the same question, here you go! Its easy!
Go to this link:
http://www.mylivesignature.com/mls_wizard1_1.php
and follow the signature wizard instructions.
Once you get the signature you want, then choose 'want to use this signature'
then choose the 'generate BB code', then 'generate code for my hand written signature'. When the code is created, highlight it and copy it to the signature spot in your profile