Start without ankle weights and build up repetitions. Then when it seems too easy, add ankle weights (start with 500g).
If you have problems viewing the video, here is a set of still images to show you the exercise.
|Sit comfortably on a chair.|
|Slowly straighten the knee.|
|Hold it there for 10 seconds.|
|Slowly lower it.|
|Repeat the exercise with the other leg. Repeat 10 times.|