Though this game is not so difficult since the developers didn't provide us lots of hints, I'm going to recommend you to do this thing which is very helpful indeed!
If you want to complete successfully all missions in the game, then do the following thing. First, enable the cheat mode and press Shift + C. Thus you will accomplish the current mission. Note that if you do this at the very start of any mission, you will be able to complete the game with a "Silent Assassin" rank.