Like "Mercy" - complete a level without killing a guard - this is probably easiest on the second level. You should be able to restart any checkpoint when you're detected (red alert only - guards can get the yellow icon over their head and you're fine). (*see note below)
It's also doable on the 11th level, "Set to Flight". I found that going through that level without killing anyone and without being detected was the fastest way to get all of the points needed for three stars. So if you have trouble on level 2, just wait and try that.
If you're still having issues, there are two even easier ways. The easiest is to wait for your second playthrough. I strongly recommend this approach. It's really trivial your second time through. You'll see why, but if you're impatient:
Alternatively, you should get the Path of Wisdom costume for completing "Dosan's Tale," a single mission that was originally the DLC for the 360 game (access with RT when selecting a specific chapter). The Path of Wisdom costume changes your stealth kills to knockouts, so you can just knock out everyone in the level as you go. The first level is a good place for it. Just make sure you equip non-lethal equipment.
NOTE: Note that GrmRpr69 says in the comments that he tried the Path of Wisdom and did not get the achievement, having to reset the checkpoint a couple of times. LEGENDXKILLA II reports that they also did not unlock the achievement when they reset the checkpoint on their way through. However, I re-ran level 2 on a new profile with Path of Wisdom equipped and had no issue unlocking Ghost even after resetting every time I was spotted (I did not kill anyone, even before a checkpoint reset). Try these methods at your own risk, I guess.