Particularly if your eggs are brown or blue/green you want the strongest light source you can find. This is often a flashlight of some sort since their beams are or can be focused. Now you want the room you are candling in to be as dark as possible. We usually wait until after dark to eliminate the sun coming in from the windows. Now place the egg in front of the light. You want to work it so that NO light can be seen except that which is actually coming through the shell of the egg. This can often be done by shaping your hand around the egg and the light, but you can also fashion a cone out of paper, a toilet paper tube, or some such to accomplish the same thing.
With the room completely dark and no light leaking from around the egg you can often candle all but the darkest of eggs.