There are a few ways to do this exercise.  First you want to get into the dead bug position:

Once you’re in this position, you’ll want to do a deep core breath, engage the core and then create resistance between your hand and your knee to challenge the core (careful not to challenge it too much).  You can repeat this on both sides, and even crossing the arm to the opposite leg.  Once you are proficient in these, you can move on to the dynamic dead bug: