The main muscle of your core is the transverse abdominal, which runs across and connects the two boney landmarks.
You can engage it by collapsing your midsection inwards, so that the bones are being pulled to come closer together.
Try to imagine making a crater in your midsection.
Some of you may recognize this muscle as the "zipper" muscle. The muscle you contract in order to pull up those tight jeans/trousers you barely fit while trying to zip up or button.
You should feel some tiny muscles in your low back kick in to relieve some tension in your back/pelvis. Now return to the neutral position and disengage your transverse abdominal muscle.
If you can’t find it this way, another good way is to make a hissing sound like “ssssssssssssss”; imagine you were letting the air out of a tire. Once you let all the air out that should be the point where your transverse abdominal is contracted. Now try finding this position again, without the hissing as you may be compensating with you’re a respiratory muscle.