Launch the Settings app on your phone. This can be found in your App Drawer, or by Pressing the menu button on the outside of your device.
Check the box for USB debugging.*
*You can also Enable, Stay Awake and Allow mock locations to make things easier when you are running tests.
Your phone should now be in Debug Mode, if you go into the notification bar you’ll find an Ongoing alert stating USB debugging connected. Note that even while USB debug mode is enabled you can still Mount your Android phone to your computer to transfer files without causing any problems!