The hip anatomy should permit a minimum of 10 degrees extension, 120 degrees flexion, 30 degrees internal rotation, 40 degrees external rotation. Now, the shape of the hip capsule and ligaments dictates that as the hip approaches full hip extension, the anterior capsule tightens, pulling the hip into more internal rotation. So, for a mobility drill to achieve an increase in hip extension range of motion, hip internal rotation is a natural permission - extend with a neutral or slightly internally rotated hip to permit more hip extension. Also, activate the core to stabilise the trunk when mobilising hip extension - to do this, lock the elbows, press down on the front knee - this activates the pecs and obliques to bring the ribs closer to the pelvis, thus preventing lumbar extension with this mobility drill - this isolates hip extension. Use breathing strategies to permit further range of motion.
The second element is that of increase external rotation mobility of the anterior capsule, whilst in some extension - the key movement is abduction/external rotation, adding extension as tolerated. Core activate as above.
Be gentle on yourself - pushing into pain sends a signal to the nervous system to resist.