Don't sit. Stand on your sitting bones.
This person is not sitting on her sitting bones. Her pelvis is tipped backwards and the natural curves of her back are pushed backwards to counterbalance the weight of her head which is drooping from her upper back. Her neck is contracted and shortened.
If you recognise this habit in yourself try sitting on a hard flat surface and sliding your hands underneath your buttocks so that you can feel a bony pressure on each hand. This is the base of your pelvis. Slide your hands out from under you in a backwards direction. Think about the weight of your torso and head transferring straight down into the surface through those bony bits (your ischial tuberosities, or 'sitting bones'). The base of your pelvis is now holding your weight as your feet do when you stand.
It shouldn't feel like hard work, especially in the mid-back.
Check in with your breathing: are you noticing any ease in your lower ribs? If that area is tight then try tipping forward on your sitting bones and coming back again to find an easy equilibrium.
Don't think you have to find one 'perfect' position, allow for constant small movements and changes of position.
Don't be upset if your former habit of sitting takes over; just keep coming back to the thought of your sitting bones and your head and your free breathing when you remember.
Whatever you do, don't fall into the 'sergeant major' trap of thinking that it's good to pull the shoulders back and 'sit up straight'. This will become uncomfortable very quickly and is putting strain on the lumbar spine. As with the 'slump' the weight of the body is not being transferred directly down through the sitting bones.