Was your instructor with you in the debrief?
From what I have seen, most examiners don't fail someone on ONE item. It is usually a combination of deficiencies.
Did he really fail you on ONE item? Or were there multiple items, but you feel that you were taught differently on one of the items that he debriefed?
It the latter is true and not the former, then I would not appeal at all, but rather you should go back and learn from the situation. There is usually more than one right answer in aviation, and if you can explain why you did something a certain way that is acceptable to the examiner, then they will usually accept it. If your instructor taught you something that is completely wrong and unsafe, then you should be having a talk with your instructor.