Bring your bum right into the back of the chair and have your rib cage above your pelvis. Stick your bum out ever so slightly, so there is a slight curve in your lower back.
Your head should be above your ribs and NOT sticking forward. To maintain this you can focus on bringing your chin into the neck slightly and your eyes should be looking forward.
Your shoulders should be parallel to your body and NOT protracting forward.
For many of you it would be a good idea to consider changing your desk station set-up, i.e raise your computer screen so the middle of your screen is eye level and bring your keyboard and mouse closer towards you so you don't have to reach forward with your arms.
It will be hard to maintain this position all day, so I suggest to focus on this position for 10-15 minutes every hour to begin with then increase the amount of time when it becomes more comfortable.