Hip flexors are really important in a lot of motions such as kicking, running, stepping and bending. 

Make sure you’ve started at the start of this series here and worked your way through:
74 – Hips: Hip Flexors Lying With Gym Ball

And you have correct posture in standing as shown here:
73 – Hips: Correction Of Sway Posture In Standing

Once you’ve got great control in this exercise for 30 repetitions, you can move on to adding a band here:
77 – Hips: Hip Flexors In Standing With Band