For ex: "I confused wave speed with transverse wave speed. Wave speed depends on the medium and is constant for any given medium. Transverse wave speed is a fn v = y'(x,t), NOT a constant."
Usually, I just identify why I got the Q wrong- did I misread the Q? Confuse 2 concepts? Misunderstand the experiment in the passage? Also, I make sure to elaborate on the correct concept/reasoning/formula needed to solve the problem.
But, really, it's however you want to do it. This way works for me.