3 steps to sitting correctly:
1: Find a chair that allows you to sit with your knees at a 90 degree angle with your heels flat against the ground.
2: Slide your bum down the back of the chair until your bum is directly in the corner of the chair.
3: Maintain the position of your bum and lean back against the back rest. You should find that there will be a slight arch in your lower back. This will help to protect your spine and will decrease pain and discomfort often associated with sitting for long periods of time.