From Yoga Journal:
Step by Step
Stand in Tadasana. Inhale and raise your arms perpendicular to the floor. Either keep the arms parallel, palms facing inward, or join the palms.
Exhale and bend your knees, trying to take the thighs as nearly parallel to the floor as possible. The knees will project out over the feet, and the torso will lean slightly forward over the thighs until the front torso forms approximately a right angle with the tops of the thighs. Keep the inner thighs parallel to each other and press the heads of the thigh bones down toward the heels.
Firm your shoulder blades against the back. Take your tailbone down toward the floor and in toward your pubis to keep the lower back long.
Stay for 30 seconds to a minute. To come out of this pose straighten your knees with an inhalation, lifting strongly through the arms. Exhale and release your arms to your sides into Tadasana.
Benefits of Chair Pose
You can increase the strength of your thighs by squeezing a block or thick book between them during the pose.
To help you stay in this pose, perform it near a wall. Stand with your back to the wall, a few inches away from the wall. Adjust your position relative to the wall so that when you bend into the position, your tailbone just touches and is supported by the wall.
Chair pose is challenging and can be uncomfortable. This is a great posture for reminding ourselves how strong we are and that we can persevere even when we are doing something we would rather not be doing. You are stronger than you think. As the sanskrit translates, the pose means powerful and fierce. Use this posture to help you learn the strength of your body, your mind, and your spirit.