DO point out and go into as much detail as possible as to why you feel something is not working. More than anything, your reasoning is critical to solving the problem.
DO tell us why we’re wrong about certain design and development decisions we’ve made. Part of the process is finding those holes.
DON’T mock up designs or alterations to our designs or code in photoshop, word, or any other program. Doing so is counter productive because we then must reverse engineer the whole thing to find out what you were trying to solve. This results in lost time, and budget.