To hide scars from self-injury, one thing that you can do is wear 3/4 length shirts. You can also wear arm warmers. They are sold at places like Hot Topic. You can also wear chunky bracelets and watches. This will cover most of your arm and it works if you are not going to be directly interacting with people, for example, going to the mall. Another tip is to stock up on long sleeves in the fall. Another thing you can do is get some tank tops and layer them with button up long sleeved shirts. In the summer time you can also wear a tunic. They are made out of thin material.