Just below the classifieds you'll see the Member Feedback forum. To post feedback you'll have to search for the name of the member you're leaving feedback for to find his/her "thread". You then post your feedback of the member in his/her thread.
To add the feedback link to your signature you'll need to edit your signature and add something similar to this:
(put proper bracket here)URL="http://www.okshooters.com/showthread.php?156241-getchurgun"]Getchurgun's Feedback[/URL]
I left out the [ before URL but that's the link for your feedback. You can change the part between the bracketed URL to whatever you want.