As a DSLR user, you actually see through the lens, which is why it is easy to use a circular polarizer.

First is to know when to put it on and when not too:

• It is not advisable to keep it on constantly, although I met people who do that, because it gives you 2 stops less light. This means your camera either uses slower shutter-speeds which makes things that move blurry or higher ISO which makes image grainy.
• Polarizers are not useful in dull or diffuse light.
• They are useful in bright directional light and you should see the effect right away when it is. Even in bright light, their effect is strongest at 90-degrees from the sun. If you are shooting into the sun or directly away, there will be very little difference.
• Polarizers are not recommended for very wide-angle lenses, because of the previous point. In other words, if you get into your frame light that is parallel and perpendicular relative to the sun, the polarizer will only affect part of the light and your results will look very strange.

Second is the how part:

• Looking through the viewfinder, frame your shot, zooming if necessary, then rotate the outer ring of the polarizer. Make sure it is tight enough otherwise, you'll unscrew the whole thing.
• Rotate the ring until you see the most pleasing image. Generally the sky gets darker to a point and then start brightening up. At the darkest point, the effect is maximal but you may not want to go that far if it makes things look unnatural, such as the sky being much darker than the foreground.
• Polarizers also remove reflections. If that is the goal, rotate the ring until you see the least reflection where you want it to go away. It is rare that it disappears entirely unless the surface is completely flat.