Setup and Key Actions
From tadasana (feet together or inner hip-width—roughly four to six inches or “two fists’ distance” apart), center your weight over the arches of your feet. Line up the back of your head with the back of your pelvis. Bring your hands to your hips, or inhale your arms up overhead. Ground down into the mounds below your big toes and your outer heels. Press your feet apart from each other like you're stretching your mat in two, and maintain that action. On an exhale, hinge forward from your hip creases (bending your knees here is fine!). Keep your spine long as you hinge forward; avoid leading with your chin, and keep your belly active. Press the tops of your thighs back as you fold without shifting all of your weight into your heels.
Bring your fingertips to the floor (or blocks), and press into them to help engage your deep core muscles. Keep your weight centered over the arches of your feet and continue grounding down into the balls of your big toes and your outer heels. Only straighten your legs to the point that you feel the stretch in the middle part of your hamstrings. If you mostly feel the stretch or pull up near your sitting bones, that's an indicator that you've straightened your legs too much. Relax your head, face, and jaw.
Place your hands on blocks if your fingertips don't comfortably come to the floor. If rounding your spine is contraindicated for you, stick with a half forward bend (ardha uttanasana or “flat back”: fingertips to the floor or blocks, arms straight, knees bent, and spine extended).