You rate the sig of the most recent post on a scale of 1-10 or 1-5 stars.
If you really want to want to comment on a person who posted more than one post up, make sure you rate the sig(s) of every user below that for courtesy.
Example: John Doe just posted his sig above me, but I really really want to comment on Jane Doe's sig which is 2 posts above me, which means that if I am going to comment on Jane's sig, I must also comment on John's sig, and I must distinguish between the two, like so:
Jane: 8/10 I really liked the way you blah blah blah...
John: 7/10 It's nice, but I didn't like the blah blah blah...
Also, you may post multiple sigs.
Let's get started. The next poster will rate my current sig.