Go to the user's profile and click on the 'Wi-Fi Trade Rating' tab, there'll be an option to submit feedback there. (Or if you're on a board where their total feedback number is displayed in the sidebar, you can click on that to go to the feedback page as well)
To give someone a positive feedback, all you gotta do is go to their profile, then go over to the " Wi-Fi/Trade Rating " tab, and click " Submit Feedback For [insert username] ".
Then fill it out, making sure to set it positive (assuming it was a positive experience; negative feedback can make you look bad on here, so don't give someone a negative feedback unless they were a troll or extremely rude), then click submit, and you're done!