In order to correctly expose your film or CCD there are two variables that must be identified: average reflectance and average light. Unless you have some good reason to meter for other than these two averages, stick meticulously to the method outlined below when you take a light reading.
This tutorial deals with reflected light readings. This is the type of light reading taken with an in-camera light meter or a hand held meter which is pointed toward the subject. This tutorial does not deal with incident light readings which are a measure of the light falling on the subject.
A white painted wall, snow or the ocean reflect most of the light falling on them. A burnt tree, a black fireplace or a coal mine reflect little of the light falling on them. A mid tone falls halfway between these extremes and reflects 18% of the light falling on it. The ISO rating of film or a digital CCD is set so this mid tone is exposed as a mid tone.
Identify a Mid Tone
Identify a mid tone for average reflectance and meter off that. Look for some green grass or foliage, mid tone rocks or bare dirt, weathered timber or whatever you can find that is somewhere in the middle between dull black and shiny white. If you are unable to escape a predominance of one extreme or the other in your framed image, then find an area to meter else where.
If need be, point the meter toward yourself and meter your clothes or else take your coat off, throw it on the ground and take a light reading off that.
Sand at the beach, or the palm of your hand are about one stop too bright. These can be used to meter off but you must compensate and the likelihood of a mistake is introduced.
Average light is half way between light and shadow. If it's overcast there isn't so much difference between the two extremities as when it's sunny but you still should take a reading of the average light.
Identify Average Light
If you are metering off some trees and scrub in full sun you'll probably find that the scene makes it's own shadows within the foliage. Be careful because the amount of sun and shadow can be confusing.
If metering a tree, aim your meter such that you read off half sun and half shadow on the tree trunk, then if need be, move around the tree to your chosen photo spot to capture the image. On a rough barked tree such as a pine tree, make sure that the texture of the bark on the sunny part has not created half shadow.
If metering a building, meter off a corner so you get one wall in the sun and the adjacent wall in the building's own shadow. Alternatively, take a light reading off half the sunlit building and half the shadow on the ground.
When photographing the landscape I always take my own shadow with me. Yes, as long as the sun is shining my shadow will be there with me. I crouch down and take a light reading off the ground pointing the meter so half of it reads my shadow and the other half reads the sunny area. That's my light reading for the foreseeable future, till the light changes. If I have any doubts about the average light reading, I take a reading from the shadow area and another from the sunny area, then see if my average light reading is in the middle.
Take a light reading Indoors
When photographing indoors by window light, be sure to take a light reading for the average of the subtle difference between light and shadow. This diffused, directional light, according to my view of the world, is the most attractive and useful light for photography. Although the level is low, the quality is high. Strive to meter it accurately.