1) Start in a push up position and raise yourself so that your hands are directly under your shoulders.
2) Inhale and engage your core. (To engage your core: pull your naval towards your spine, tighten your ribcage towards centre, draw in the muscles between your two hip bones, feel a gentle lift in your pelvic floor).
Squeeze your glutes!
Make sure your head is in a neutral position - do not raise your head or let it drop to the floor.
Your whole body, from head to heels, should form a straight line.
3) Keep your core engaged as you exhale. Repeat breathing and hold position for 30 seconds or until there is discomfort in the lower back.
Here's a link to a study comparing muscle activity during different plank exercises:
If you struggle with this plank, try one or both of the planks pictured below until your strength increases.
As you get stronger, aim to hold your plank for longer.