Which Side Is Anakin's Scar On? His Right Eye

Anakin Skywalker's scar is over his right eye. Face him head on and that's the left side of your screen, so "his right" and "your left" are the same mark. (Revenge of the Sith; Wookieepedia, Anakin Skywalker)

Which side is Anakin's scar on?#

Wookieepedia's canon article says he received a scar over his right eye during the Clone Wars. It's the one you see in Revenge of the Sith, and it's why the question trips people up: "right eye" is the character's right, and the screen shows it on your left. (Wookieepedia, Anakin Skywalker; Revenge of the Sith)

The Legends article is more specific. The Dark Horse comic collected in Star Wars: Clone Wars Volume 6: On the Fields of Battle gives him a vertical scar running above and below his right eye, plus two small scars on his left cheek. (Star Wars: Clone Wars Volume 6: On the Fields of Battle)

Which eye is it, and does it touch the eye itself?#

Same answer as above: the right. The wiki wording in both continuities is "over" or "above and below" the eye, so it's a mark on the face around the eye rather than a blinding wound. I'd call it a brow-and-cheek scar, and that's an observation from the wording, not a quoted fact. (Wookieepedia, Anakin Skywalker and Anakin Skywalker/Legends)

When does he get it?#

Canon doesn't pin it down. In the comic 501 Plus One, set at the Battle of Arantara, Wookieepedia has him without the scar, and he has it by Revenge of the Sith. So it arrives some time after that battle. The films and shows don't explain it, but the article says it's believed to have come from Asajj Ventress, citing a Japanese reference book, the Star Wars Battle Encyclopedia. (501 Plus One; Revenge of the Sith; Star Wars Battle Encyclopedia)

Legends names the cause: Asajj Ventress, in a lightsaber duel. The full story is on how Anakin got his scar. (Star Wars: Clone Wars Volume 6: On the Fields of Battle)

Does the poster show it?#

No. The wiki notes that the scar is absent from both the 2005 DVD cover and the poster of Revenge of the Sith, even though it's on his face in the film. If you picture a clean Anakin on that art, that's why. (Wookieepedia, Anakin Skywalker)

Is it the same as Vader's scars?#

No. Vader's injuries came on Mustafar and are covered in what happened to Anakin's face and how Anakin survived Mustafar. The eye scar belongs to Anakin the Jedi general during the Clone Wars. Don't mix the two up. (Wookieepedia, Anakin Skywalker)

My read: if you're drawing him, mark the right eye (your left), keep it vertical, and skip the cheek marks unless you're working from the Legends comics.