Developing a strong core is so important to posture, performance, and overall health and well being.
Here is a quick explanation of the prone plank.
Make sure the spine stays straight. (Neutral)
Keep shoulders over elbows and forearms flat.
Stay up on your toes.
Hold this for 30-90 seconds and repeat it 5 times.
Letting the hips drop can compress your spine which you do not want.
You can raise your hips instead like this
Another variation you can use is the push-up plank.
When you get comfortable with holding yourself up learn to squeeze deeper and deeper try to feel it all around your abs, obliques, and back.
Oh and by the way your core is not just your abs, it's everything in the middle of your body!!!!
Have fun!!