Skip to the 9:23 mark here for a video demonstration.
It's extremely common to find hip mobility issues in people with poor ability to stabilize their trunk.
An Active Straight Leg Raise often yields more range of motion than a Passive Straight Leg Raise; the reason being is that the person is required to stabilize proximally to the moving joint.
Use of the wall, as in the wall bug exercise, can greatly assist the person in learning to engage the abdominal wall.
This exercise can assist in patterning and improving habits through complex movements such as squatting and deadlifting.
Engage a rigid trunk through diaphragmatic breathing.
Keep the low back in the same position through the motion.