Locate the name of the member you want to leave feedback for, you can use the Member List under Tools and open up their profile, or just find their name on a post and look for the line that says 'Reefer Ratings"
Click on the number beside the 'Reefer Ratings' and it will take you to the Feedback screen.
There you will find a line that reads 'Submit feedback for (reefers name)'
Click that & fill in the boxes.