Like the achievement states, perform 20 stealth takedowns.
You perform take downs using the
button, either when you have snuck up behind an enemy or are hanging from a ledge.
There are a ton of instances throughout the game where you can use take downs on armed enemies, sometimes up to 5 in one section.
This achievement is also cumulative, so if for some reason you never got it on your first playthrough, you should easily nab it on your subsequent playthroughs.