'Open chested' is just another way of saying 'front on'.
If you are a front on bowler, your chest needs to be open to the wicket... straight line from shoulder to shoulder facing. This means you must be looking down the wicket 'between' the arms.
If you are a side on bowler... straight line from shoulder to shoulder but in-line with the wicket. This means you must be looking down the wicket behind the front forearm [the left if you are a right hander].
If you are a front on bowler, your chest needs to be open to the wicket... straight line from shoulder to shoulder facing. This means you must be looking down the wicket 'between' the arms.
If you are a side on bowler... straight line from shoulder to shoulder but in-line with the wicket. This means you must be looking down the wicket behind the front forearm [the left if you are a right hander].