If you're just tuning in, be sure to check the links people have provided. They show plank variations, progressions, etc.

One small warning: the woman trainer pictured on one site is looking upward with her chin raised and her neck scrunched. Don't do that. Keep your head, neck, and spine in a neutral position: all in a line. You can look a little way forward with only your eyes if that helps. But keep that neutral spine.

And remember to breathe. Yes, you are pulling in your abdominal muscles, but you still need to breathe if you're going to make the distance.