1.  In the bottom right corner of your screen click the pink button labeled "Sign out" 

2.  Your desktop will then darken a bit and the "Exiting session" window will appear, go ahead and click the blue "Sign out now" button.

3.  Now press and hold the power button on your keyboard

4.  A small window should open up on your desktop (options may differ from screenshot), click the "Power off option"

Once the Chromebook powers down give it about 30 seconds before pressing the power button on the keyboard to turn your device back on.