Back leg: hip flexors, quadriceps.
Place one foot at the front and slide back with the opposite knee. Allow your hips to slowly relax down against the floor, without pushing. Just drop your hips, and relax from your navel to your toes on your back leg. Play with the front foot – meaning play around with different ways of placing the front foot and leg, to support you in feeling the target area: hip flexors on your back leg. No 90 degrees between front foot and knee is a must here, just play around with that front leg. Drop your knee out to the side if you want. Drop your foot out to the side if that supports you. Rest your arms on the floor if that´s the best version for you, or use blocks as in the picture.
There´s not one way to do this pose. Feel the target area, and modify the pose until you feel the target area, and can allow your muscles to relax.
Stay around 2-5 minutes. Then slowly ease out of the pose and relax a moment before you shift leg.