Start by cutting the desired end shape out of the sides. Remember not to go past 4″ on the vertical (up and down), as the top and bottom shelves will sit at 4″ from the top and 4″ from the bottom. After you cut one side out, use it as a pattern for the other side.
Now would also be a good time to mark out your shelves, as shown below, on the sides. Remember, the shelves are 3/4″ thick.