First, select a few traits that are prerequisites for success in this position (technical proficiency, engaging personality, reliability, and so on. Don't overdo it — six dimensions is a good number. The traits you choose should be as independent as possible from each other, and you should feel that you can assess them reliably by asking a few factual questions. Next, make a list of those questions for each trait and think about how you will score it, say on a 1-5 scale. You should have an idea of what you will call "very weak" or "very strong."