Start with hands directly underneath your shoulders in high plank position. Ensure your core is activated and belly drawn in. Go down to low plank, by dropping to one arm at a time onto your forearms; push back up to high plank, one arm at a time, and repeat.
Ensure - your body and back forms as straight and as flat a line as possible / your neck is in line with your spine / act as though you're pushing the ground away through your triceps each time you return to high plank / hands stay facing forward on both the way up and down/ engage your abdominals to assist you in bringing yourself up and down between high and low plank / throughout the exercise, aim for as little movement as possible at your hips and torso.