To begin sit upright on your 2 sit bones on the chair, cross one foot over the other thigh and lean forward 

Ensure you lean forward from the hip. You should feel the stretch around the back of the hip. 

Next  place one hand onto the opposite knee, and pull knee up and across as far as you can. turn your body to the side, towards that knee. 

Ensure  your shoulders are level 

Try to image you are rotating around a skewer that runs down the length of your spine. 

Aim  to have both sides feeling the same. If one side is tighter, spend longer stretching this side. 

Hold for 10-15 seconds, for 3-5 repetitions on both sides.