Get into a kneeling lunge: one leg forward with the toes pointing forward and the foot flat on the ground, and the back leg's knee on the ground and the toes pointing back.
Move your front foot forward enough so that you can bring your hips down without the front heel coming off the ground.
Aim to bring your hips as close to the ground as possible, getting to a 180 degrees angle between your front and back thighs.
Keep your chest up to intensify the stretch on the back leg hip flexors, and keep facing forward (don't let your hips twist to one side).