To get into position, sit your butt back just like you are about to sit in to a chair. It is very important that it is your butt going backwards; you want to avoid your knees going in front of your toes because this can cause damage to your knees. Try to create as close to a 90 degree angle at your knees as possible, or think about bringing your thighs parallel to the floor.
Keep your upper body as upright as possible. and raise your arms in front of you, reaching for the ceiling. Lengthen through your spine.
Now that you are in Chair Pose, begin to do squat jumps, maintaining the Chair Pose position as best as possible each time you jump.
To make this a nice mix of plyometric and isometric (working your muscles by holding them still in a flexed position), try this:
Do 12 chair squat jumps and at the end of the 12th jump, hold the chair position for a count of 10. Repeat this for 1 minute.
You'll feel this exercise in no time!