Setup and Key Actions
From Plank, lowering down with control, keeping the elbows tucked close to the body, you can have knees down or knees up, remembering to tuck in the tailbone not allowing the bellybutton to sag towards the ground, keeping the spine long not allowing the lower back to bow. Try to visualize your body in one long line from the crow of your head to your knees or heels of your feet.
Modifications
Lowering the knees to the floor if you are just starting out or need to gain more control or have any lower back issues, starting with the knees down will help to build your core strength so you can stay away from injury and work towards lowering down as one long line with full control.