How to take a neter reading off of grey card?

So I have a nikon 3100 and I need to take a grey card photograph and i'm cluless. Can someone help!

Fill the frame of the camera viewfinder with the grey card.

Make sure that the light falling on the grey card is the same as that falling on your subject.

The shot does NOT have to be in focus.

NOTE: Your instructor should have given you explicit instructions on how to do this.