You will have to think in the sagittal plane in order to teach your client this stretch.
As you know the sagittal plan divides the individual in to right and left parts.
With either the right or left part, have your client place (let’s say for the sake of learning the stretch) the right buttocks and thigh on the side of a chair while the left side of the body (the left buttock, thigh,leg are not supported by the chair)
Essentially you are seated on one side of your body.
Take the left leg and extend it behind you with the objective of directing the knee towards the floor. Squeeze the left glute as you perform this exercise and repeat on the opposite side.
You can also perform a stretch for the iliopsoas in this position.
Hope this helps!