From a standing position, bring the body forward with a great stride without lifting the back foot off the ground. From this position, flex the back knee transferring most of the bodyweight to the front leg. The front knee must remain in a position just above the foot, never going beyond it. You can gently lower the weight of the trunk vertically by bringing the pelvis toward the floor to increase the stretch.
With this simple exercise we can do really work the hip flexors. To keep balance, you can rest your hands on the front leg or a side bench, as balance is vitally important in order to be able to perform the exercise correctly.
If we had to choose only one exercise to stretch the illiopsoas, this would be it, because of its simplicity and effectiveness.