Drawing a manga body on your computer is not that different from how you might draw a regular person. Manga bodies tend be slim and long, having an appearance similar to a person you would say has a light build. The best way to begin is by sketching broad guidelines with a 30% black pencil. After that, you want to draw simple sticks for the arms, then intersecting curves to start the chest and a V for the stomach. Follow the example in this video to see the rest.