There are probably a dozen variations of a plank exercise but before you can do even one of them, you need to learn how to do a proper plank.
Planks are great for building core stability. You can do them on your hands or elbows; on your toes or knees. Once you’ve mastered the basic plank, you can add some variations.
Remember to keep your legs steady, tighten your abs and don’t your wrists under your shoulders. Keep your neck neutral and don’t tilt your pelvis.
Here’s a video demonstrating how to do the plank properly.