A popular trend in the '80s, sweatshirts were often altered to fall off the shoulder. The look is said to be inspired by the popular 1983 film "Flashdance" starring Jennifer Beals, in which the main character often sports the look. While many stores sell sweatshirts that are done in this style, the look can be created at home using a basic sweatshirt without a hood and scissors for the same effect.
Turn the sweatshirt inside out. Use the permanent marker to trace a line that follows the neckline, roughly 2" from it. Keep the line as straight as possible all the way around the sweatshirt.
Cut along the line with the scissors.
Flip the sweatshirt right side out and try it on.
Repeat the process, cutting off only 1 inch at a time, if it doesn't fall of your shoulder as much as you would like. It is easy to cut off more, but you cannot put pieces back once you've cut them.