Set the ISO of your Canon EOS Rebel XTi to 100 and choose an outdoor subject. Activate the Spot metering/Partial Metering option on your Rebel XTi and set your camera’s mode to manual and to the shutter speed to 1/125. 

Observe your subject and break it down into its simple subject (single tone) components. Choose a Reference Tone.  A Reference Tone is a simple tone (one tone) usually chosen from the most important part of the subject. Once you have selected the Reference Tone, point your spot meter circle at this tone so that the spot frame falls WITHIN this simple tone and find its normal exposure. If your subject does not have a distinct single tone pick out a tone that is medium gray or brighter.

Let’s assume the "normal exposure" indicated by your camera for this Reference Tone is 125@f-8 (i.e., 1/125 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 (or a Grayscale density of 128 (out of 256) in photoshop on your home computer when you convert the image to black and white/grayscale).
The image tone that your camera's normal exposure provides is independent of the original tone of your subject. YOU MUST UNDERSTAND THIS since this is the foundation of the Zone System of Exposure!

In Zone System/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. Decreasing the exposure time  by two stops converts the 18% gray image tone to 4.5% Black (028 on Gray Scale). The correct exposure is now 1/125@f-16 or equivalent.

If you chose Dark Gray, you need to decrease the "normal exposure" setting by one stop.  Decreasing the exposure time by one stop converts the 18% gray image tone to Dark gray (078 on Gray Scale). The correct exposure for this is 1/125@f-11 or equivalent.

If you chose Medium Gray, leave the aperture and shutter speed as they are since the meter is already creating an 18% gray (Medium Gray - 128 on Gray Scale) image tone, i.e., the tone of the image matches 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. In this case the correct exposure is 1/125@f-8 or equivalent.

If you chose Light Gray, you need to increase the "normal exposure" setting by one stop. Increasing the exposure time by one stop converts the 18% gray image tone to Light Gray (178 on Gray Scale). This results in a correct exposure of 1/125@f-5.6 or equivalent.

If you chose White, you need to increase the "normal exposure" setting by two stops. Increasing the exposure time by two stops converts the 18% gray image tone to White (228 on Gray Scale). The correct exposure would be 1/125@f-4 or equivalent.