How: The scar is from a killing curse that somehow didn't work, backfired, and "killed" the creator of said spell, the villain of the entire series, Voldemort.
Location: Both sides of his mouth...in the terrifying position that you can imagine exactly what had to happen to create them.
How: We never really discover that for certain. The Joker has some serious reality issues and so he tells different tales on different occasions. The official story is that he fell in a vat of chemicals. His own stories in this particular film are that his father did it to him, and that he did it to himself to make hiw wife feel better.
How: Perhaps only Tarantino can tell you that one, as it was his intention to leave it as a mystery.
Random Nazis
Location: the forehead of any Nazi or German soldier with the misfortune of running into the Basterds.
How: A big ugly knife weilded by one of the Basterds. they mark all of the Nazies they allow to live to be able to humiliate them and recognize them for life.
How: Completely justified, in my humble opinion. High on a cocktail of PCP crystal meth and LSD, Mason (a particularly terrible child molester) is convinced by Hannibal Lector to cut of his own face with a piece of glass, feed the majoirty of it to the dogs, and eat his own nose.
Location: missing a tooth, scar on forehead, scar on right calf, and one on the left forearm.
How: no explanation for the tooth. The forehead is from a fight on St. Patrick's Day. Scar on leg is from a thresher shark. The one on his arm is a tatoo that he had removed.
Hooper
Location: scar on left forearm and on right leg.
How: scar on arm is from a Moray eel that bit through his wetsuit. Scar on leg is from a bull shark that scraped him while he was taking samples.
Location: Prominent all over face, but more centered around the chin.
How: We never get the back story to this one, but judging by the fact that he is a VERY angry war worn sargeant in the middle of the Vietnam war, I'm going to guess that it had something to do with warfare.
How: Another one without explanation, but they are lions! It's pretty safe to assume that paws and teeth...and perhaps sibling rivalry...were involved.
Location: Line running all around his left arm and shoulder indicating a total replacement of that limb.
How: He was involved in a car accident in which his car and another ended up in the river. A robot pulled him out and saved his life, leaving a little girl to die in the other car, feuling his hatred of robots. His entire left arm was replaced with a cybernetic one.
Location: All across his face and across both cheeks, slitting them open and having to be held shut by piercings. Also...the tongue is pretty much gone.
Location: Several scars all over his body, the most noticable of which is a scar on just below his left eye, and the largest of which are on his back and arms.
How: A quite strange assassination attempt involving bilhooks, and a lot of nude fighting.
Location: Three slash marks running from his right cheek to the back of his head.
How: A "gift" given to him by the natives on his first day on the planet Pandora. He had the options of having it removed, but chose to keep it as a reminder of what can happen at any time.
Location: Vertically down right cheek, distorting features including the right eye.
How: Please correct me if I'm wrong here because I personally despise all James Bond films, but as far as I can tell, it is only classified as a dueling scar, possibly from Mensur fencing.
Location: Vertically from right eye down to right jaw line.
How: Again not known, though it could have occured during the car accident that separated him from his brother, Austin Powers, or when he was "placed in a burlap bag and beaten with reeds."
http://www.listal.com/viewimage/1764190