You want the chair to remain firmly in one place. Your feet should be planted on the floor, with your knees bent at ninety degrees.  Make sure the chair is not wobbly, and doesn’t rotate or have wheels (e.g. an office chair) If you are at the gym, you can sit on a weightlifting bench. The exercise band should be placed behind your knees, on top of your clothing. The band should be tight enough so that it is challenging to pull your knees apart, but not so hard that you cannot perform the exercise. Alternatively, you can also create resistance using your hands. Simply make a fist with both hands and place each hand on the outside of each knee. For example, your left hand will be on the outside of your left knee, and the right hand will be on the outside of your right knee. Use the strength of your arms as resistance. Take a breath in, and as you exhale, begin the exercise. try to push your knees apart, against the resistance bands for four seconds. Then, exhale as you release.  Keep the muscles in your abdomen contracted while you do this exercise to keep your spine protected. Don’t hold your breath as you perform the exercise, as this may make you dizzy. You should try to press against the band so that you feel the outer muscles of your thigh contracting. It is not necessary to spread your legs as wide as they will go. If it is possible for you to do this, you might need to try a tighter resistance band. You should try to do three sets of 15 repetitions. This means that you will repeat the exercise 15 times, take a short break of a minute or so, and then repeat for two more sets of 15. You should aim to do this exercise three to five times per week.  If you feel that this is too hard for you, then do as many as you can, and work your way up. As you get stronger, you can move to a tighter resistance band.

Summary:
Sit in a chair with your knees bent. Place an exercise band around your thighs. Press your knees outward. Repeat the exercise.