Step by Step Guide
Begin in a quadruped position on all fours with your hands directly under your shoulders and your knees directly under your hips. Look down at the ground so that your neck is in neutral. Engage your abdominals by pulling your belly button toward your spine to keep the spine in neutral.
Lift one knee up off the ground an inch and flex the foot. Then, press your heel straight up toward the ceiling while keeping the leg bent at 90-degrees. Raise that leg until the thigh is parallel to the ground. Be sure to engage your core to keep the hips levels instead of rotated.
Repeat for the desired repetitions, on both legs.
A common mistake is arching the back. Instead, focus on drawing the belly button up toward the spine to keep it in neutral. Then focus all the movement originating from the hip as you extend the hip and contract the glutes. Keeping the abs engaged will also help keep the hips level to avoid rotating.