Hip flexors are the muscles that lift and bend your leg at the hip. They can be a culprit for producing hip and groin pain.

Start with finding good posture in the previous video:
73 – Hips: Correction Of Sway Posture In Standing
As well as finding pelvic neutral and activating your core and abdominals with the foam roller series:
07 – Foam Roller On Floor: Positioning Pelvic Neutral

Then you can safely train your deep hip flexors with this video.