Take a Dead bug position, with only the lower back in contact with the ground.
Extend one leg forward while moving the opposite arm overhead.
Return your leg and arm to the starting position and repeat on the other side.
Form tips
Keep your head neutral: do not crane your neck forward. It may help to look at the ceiling.
Aim to be in contact with the ground with as small of an area of your lower back as possible. This will create additional Abs engagement.
Risks
If this exercise causes neck pain, rest your head gently on the ground or on a pillow. You might be straining your neck unnecessarily. Once you are comfortable with the exercise with your head resting, lift it slightly off the ground, taking care to keep your neck neutral (no craning forward).