Note: the plank series calls for a two-point plank on your elbows, but you can also try the move from a high plank (as seen here) if that works better for you.
Start in a plank position. Lift your left leg off the ground so your heel is even with your pelvis.
Keeping your torso steady, reach your right arm forward. Really brace through the abs by pulling your navel to your spine. Hold this position for 15 seconds. Repeat on the other side for 15 seconds.