Set the ISO on your Nikon D80 to 200 and choose an outdoor subject. Activate the spot metering option on your D80 and set your camera’s mode to manual and to the shutter speed to 1/250 sec. To avoid confusion, on this page we will increase and decrease the exposure by opening-up and closing-down the aperture. The shutter speed will remain fixed.

Observe your subject and break it down into its simple subject (single tone) components. Choose a Reference Tone. A Reference Tone is usually chosen from the most important part of the subject. Once you have selected the Reference Tone, point your spotmeter at this tone and find its normal exposure. If your subject does not have a distinct single tone and you are using slide film, pick out a tone that is medium gray or brighter. If you are using a negative film, pick out a tone that is medium gray or darker (remember: for this ignore the color and consider the simple subject's gray-density TONE - Please see illustration).

Let’s assume the "normal exposure" indicated by your camera for this Reference Tone is 250@f-8 (i.e., 1/250 sec. at an aperture setting of 8).
As you may know, the spotmeter’s "normal exposure" from any simple (single toned) surface will always provide the photographer with an 18% gray image tone (a Gray Scale Density of 128 where black is 000 and white is 256 on your computer monitor).
The image tone that your normal exposure provides is independent of the original tone of your subject. I know it does not make sense but you MUST understand this before you can use the technique

In spot metering, converting the "normal exposure" to the "correct exposure" is what a photographer must do. In other words, the "normal exposure" readings of the spotmeter must be interpreted by the photographer to determine the subject’s "correct exposure." The principle behind this simplified technique is that if one tone of a complex subject is exposed correctly, the rest of the tones follow and will also be correctly exposed . Now decide which one of the following tones would most closely matches your Reference Tone: Black, Dark Gray, Medium Gray, Light Gray or White. With this simplified technique you must choose one of these five tones. Once you have decided which one of these five tones best matches your Reference Tone, then adjust your camera settings accordingly:

If you chose Black, you need to decrease the "normal exposure" setting by two stops. Closing- down the aperture by two stops converts the 18% gray image tone to Black with a Gray Scale density of 038 (approx: rounded up for simplicity). The correct exposure is now 250 @ f-16.

If you chose Dark Gray, you need to decrease the "normal exposure" setting by one stop. Closing- down the aperture by one stop converts the 18% gray image tone to Dark gray with an approximate Gray Scale density of 078 . The correct exposure is now of 250 @ f-11.

                                                                                        

If you chose Medium Gray, leave the aperture and shutter speed as they are
(250 @ f-8) since the meter is already creating an 18% gray/Medium Gray image tone with a Gray Scale density of 128, then, in this case the tone of the image will match the approximate tone of the Reference Tone. Also remember that when metering from a medium gray surface, the "normal exposure" and "correct exposure" settings are approximately the same.  
  

If you chose Light Gray, you need to increase the "normal exposure" setting by one stop. Opening-up the aperture by one stop converts the 18% gray image tone to Light Gray (Light gray with an approximate Gray Scale density of 178). This results in a correct exposure of 250@ f-5.6. 

If you chose White, you need to increase the "normal exposure" setting by two stops. Opening-up the aperture by two stops converts the 18% gray image tone to White (White with an approximate Gray Scale density of 228). The correct exposure is now 250@f-4.