Find the post you want. Click quote as you normally would, then copy what is in the text box. Go to "edit signature" in user cp and paste it in there. thats how i did mine.
or... {QUOTE=username}witty comment{/QUOTE}
With square brackets obviously
EDIT: You don't get the post link doing it that way
http://shoppingsecure.com.au/ - JC's Rep
PM me or email philthy@shoppingsecure.com.au for all your stereo needs