HOW TO DO A PLANK. 1  Start by getting into a press up position. 2  Bend your elbows and rest your weight ono your forearms and not on your hands. 3  Your body should form a straight line from shoulders to ankles. 4  Engage your core by sucking your belly button into your spine.5  Hold this position for the prescribed time.OW TO DO A PLANK. 1  Start by getting into a press up position. 2  Bend your elbows and rest your weight ono your forearms and not on your hands. 3  Your body should form a straight line from shoulders to ankles. 4  Engage your core by sucking your belly button into your spine.