To do the plank pose correctly, and without modification, you will start on the ground, in table pose. Table pose is done by kneeling on the ground (or on a stable surface, like your bed), and placing your hands directly under your shoulders. Shift your weight around until you have approximately 50% of your weight on your hands and 50% on your shins. Slightly clench your glutes, and tuck your belly button towards your spine - this will help support the weight of your torso, and align the spine in the process. As best you can rotate your elbow pits forward, which will help lock out the elbow joint, and allow your skeleton to support a portion of your weight, and not just your muscles and connective tissue.