Start with your wrists under your shoulders, keeping your elbows from locking.

Maintain a neutral spine, keeping your head facing the ground.

Shift your weight onto the left hand and tap your right hand to your left shoulder. Alternate hands.

Squeeze your abs and glutes, keeping your knees from locking.

Modification:

Perform a knee plank, with your wrists underneath your shoulders. Keep your elbows from locking and knees on the ground. Be sure to keep a neutral spine.