Sitting toward the edge of a chair with good posture and knees bent, place a ball in between your knees; press the knees together to squeeze the ball, taking about 1 to 2 seconds to squeeze. You should feel the resistance in your inner thighs. Slowly release, keeping slight tension on the ball so that it does not fall. Repeat 8 to 10 times. Rest. Do another set of 8 to 10 repetitions.
Modification: For a greater challenge, change the count of the squeeze by squeezing the ball and holding for 5 seconds, then releasing again. Or, do short, quick pulsing squeezes.