You should use the simple past to indicate something that is finished on a specific date/time. Use the present perfect to talk about something that happened over a period of time up to right now. So in your first example it would be better to say, "I saw the movie" if you include when you saw it (e.g., "last Friday"); otherwise, "I've seen the movie" is better, because you're not being specific, you're just saying you have seen the movie at some point in your life. In your second example, it is likely that "he did it" once at a specific time; therefore, simple past is better. "How has he done it" implies that he has done it at some non-specific time in the past, which is probably not correct.