The best Abdominal activation you are NOT doing. •
Forget the sit-up.
Say adios to the crunch. 
The DeadBug (and its many modifications) are the way to go for isolating and re-establishing functional abdominals. •

Pay attention to your body and take your time. If you are unable to activate and engage your abdominals or you feel low back pain while doing this, then regressing and getting assessed should be your next step.