To backstab an enemy, you either need to have a pair of daggers or faeblades equipped in either weapon slot (please note that Faeblades DO NOT count towards this trophy, so grab some daggers). Then essentially all you have to do is sneak up on 20 enemies and backstab them for the achievement.
I found a great spot to grind this out was in the forests north of Gorhart. While enemies appear on your mini map, having 3 points in the Detect Hidden skill will enable them to appear as arrows, giving you an advantage to sneaking up on them. Also rolling quickly behind them using Circle while in Sneak Mode can get you in a quick position for a backstab if you time it right. I also leveled my daggers up a bit so that I could quick kill enemies in one shot. This is not a requirement, but it helps if you want to silently backstab a group of enemies.
I also had 3 points in my Stealth ability which helps a lot. Watch the icons above your enemies heads as this will help you indicate when they are close to detecting you. If you sneak up on an enemy and another sees you, you can quickly roll out of the area and usually (not always) they will leave you alone after a few seconds. Rinse and repeat til the trophy pops.
As noted on TrueAchievements: You do need at least 1 point in the Finesse skill called Assassin's Art to have the ability to backstab. Please note that all new characters are automatically given a point in this skill making it possible for any character to backstab as soon as they receive a pair of daggers.