Sit up straight on the front edge of a sturdy chair, so that your feet are supported on the floor, as shown in the first image.
Bend your operated leg back as far as you can.
Try and keep your foot flat on the floor and do not allow it to move forwards.
Now slide your bottom forwards on the chair to assist with a deeper knee bend.
As you push forward in to the knee bend your heel will naturally lift off the floor as shown in the second image. This will allow you to get more bend in your knee.
Repeat 10 times.