To come into this pose make sure to warm up your body amply with a few half sun salutations or other physical activity. There are many entries into this asana, I will lead you into it one way here.
Begin standing at the top of your mat in Mountain Pose, Tadasana. If you are unfamiliar, review further instructions on Tadasana here or seek advice of a qualified yoga instructor.
Folding from your hips drop your hands to the floor, keep your back long and chest reaching forward. Step your left foot to the back of your mat, approximately 3.5 - 4 feet back. Land your left foot in a 45 degree angle, toes pointing toward the front left corner of your mat, heel on the ground, make sure your left knee is pointing in the same direction as your toes. Keep your left foot in line with your hip. Your right knee is bent at a 90 degree angle, thigh parallel to the floor, toes pointing forward. Your feet should still be hip width apart, you can look at your feet and make sure your heels line up or slightly wider.
With your hands still on the ground, firm up your left leg by pressing your femur back toward your hamstrings. Press into the outer edge of your left foot, with this action your left hip will start to move forward. Press down into your right foot, keeping the knee bent, to press your right hip/thigh crease back.
Once your legs feel strong and secure, engage your belly and lift your torso and your arms toward the sky. As you rise, continue pressing your feet into the floor, your left hip moving forward and your right hip moving back. Keeping your front hip points and your shoulders pointing forward. With your arms extended straight up to the sky, slightly turn your pinkies inward. Biceps frame your ears, shoulder blades firming into your back and down toward your sacrum.
Hold for 5-10 breaths and repeat on the second side.
Soften your tongue away from the roof of your mouth and un-clench your teeth. Keep gaze soft and directly in front of you, if you have space to take it deeper, you may press your palms together and gently look toward your thumbs. Fill the space in your mid back with your breath and gently invite your tailbone toward the floor.