This is to help you find the most comfortable position for long periods of sitting.
1. If your chair has a back support, sit up against the support so your back is slightly curved.
2. Your upper back should be in a comfortable position, not slouched too much or too upright.
You may have to trial and error different positions.
This is key as your comfortable position may change as your sitting time progresses.
3. If your chair does not have a back support (such as a stool), try to sit up so you have a slight curve in your lower back.