There are two screws on each side of the upper legshield, just below the handlebars. The front cover, or the "grill" if you will will then come loose. Push down on the upper side, on the black plastic and it will come loose, and then slightly push in on the sides of the painted part. It will come off, but can be stubborn.
The overflow container is right there, and also remove the radiator cap when cool and make sure the coolant is up to the neck. Make sure the cap is re-installed correctly, otherwise you will have a mess. A story for another time.