This can be easily done on the second mission, which is actually the first mission that you can acquire a Silent Assassin rating.
1. Start with standard equipment.
2. Head right, and sedate the lone guard once he turns into the left corner.
3. Suit him, and head into the vineyard. Go downstairs.
4. Head to the first target, and toss a coin on the ground near the container.
5. Once target walks here, kill him, and dispose his body into the container.
6. Go up the stairs, and leave. Turn right immediately.
7. Head through the door at the end, continue on. Be careful not to run into the white steps where the guard ahead is, or you may trigger an alarm.
8. At the dead end, climb the pipe up to the second floor.
9. Enter the room, kill the second target. You can leave his body there but just to be safe, you can toss the body out of the window.
10. Climb back down the pipe, and return to your suit.
11. Suit up, and run all the way to the left of the main entrance, and then make a right turn when possible.
12. Continue on, go down the narrow ramps.
13. Sneak past the first guard near the water (or push him down if you wish, it doesn't matter).
14. It's difficult to sneak past the second guard, so sneak to his back, and push him down. You'll unlock:
15. Head to the exit, and escape.
Video below (I did this on Professional):
Sorry, this Game Clip has been removed