You find their difference, of course!
E.g. 17:15 - 13:40 = 3:35/ 3 hrs 35 min.
Just remember that a full hour (1:00) is 60 minutes (0:60). You would never write anything as 0:60, though! Anything over 0:59 becomes 1:00 + the remaining number of minutes.
Hope this wasn't too confusing! Practice makes it easier to handle and understand, so try it out! Hope this helps :)