The easiest way that I have found is to open two browser windows, one that you work in and one with the deal thread up as you will need it to leave feedback. You will need to copy/paste the URL into the required box on the feedback page.
Click on that persons screen name, click on "view public profile", click on the "feedback score" tab, in that box you will see "view complete feedback for xxxxx", click on that, under itrader profile there will be "submit feedback for xxxxx", click on that and follow what you see.
Hope this helps.