First explain the difference between your abdominal muscles and your core. Have your client stand up straight (Shoulders back and down, chest up, weight evenly distributed on their feet) and place your hand lightly on top of their head, tell them to push up against your hand with their head while trying tu pull their belly in towards their spine. This should give them a good feel for contracting their abdominals. Now have them also squeeze their glutes (With their shoulders back and down and their stomache in while pushing against your hand) this is a contraction of the core.
Stephen has a great option..another would be to have your client stand or lay down, face up and use a scenario if they were to take a minimal impact to their stomach, the natural reaction would be to contract their abdominal muscles, similar how a boxer would take a punch to their mid-section.
It’s a tough one
I’ll have my clients lie on their backs, knees bent and learn how to do a pelvic tuck while feeling their abdominal connection
I will also ask them to contract and feel the contraction
I also bring in pictures of the muscles/bone/attachments or we google them.
I talk about posture and the integration of back muscle along with the abdominals.
I use a mirror and show them
It’s not easy!
Once they become stronger they will start using thier abdominals much more.