We use "man" when we want to make a general statement about how things are done (or not done), or when we want to ask a general question, assuming that there is a right and a wrong way to do things, i.e. whenever you'd use a general "you" in English (not the "you" directed at your conversation partner):
"Das macht man nicht!" - "You (just) don't do that!"/"One (just) doesn't do that!"
(This is what parents tell (or used to tell) their children when they eat with their mouth open, when they speak with their mouth full, when they yawn or cough without covering their mouth, etc., etc.)
"Wie konstruiert man einen korrekten deutschen Satz?" - "How do you/How does one build a correct German sentence?"
"Was muß man tun, um Mäuse loszuwerden?" ("How do you get rid of mice?") - "Man stellt Fallen auf." (You set some traps)
"Wie kommt man am schnellsten von X nach Y?" - "What's the fastest way to get from X to Y?" (How do you get from X to Y as quickly as possible?)
Hope that helps!