Because lighting can vary greatly in a silhouette setting the exposure can be a challenge. There are two ways that you can set your exposure for a silhouette. How you do this depends on your chosen shooting mode. The first option would be if you are shooting in either full auto, aperture priority, or shutter priority mode. In any of these modes you would meter the scene and dial in an exposure compensation to compensate for the usually brighter background, and make the picture. The downside of this method is that depending on the situation (how bright the background is, whether or not the sun is in your composition etc.) you could be dialing in an exposure compensation anywhere from +1/2 a stop to +3 stops, and even if you do this a lot you are really just making an educated guess as to how much compensation you’re particular situation will need.
The method that I prefer is to use the manual metering mode and the spot meter of my camera. When using this method I look at the scene and try to decide what area of the scene I would like to appear as a mid-tone. In an image such as the silhouetted frog image a medium green. So I would place my spot meter on the leaf set my camera and make the image. In the case of a sunrise or sunset the spot I meter will become a medium orange color. Using the example of the mother and chick loon below I would place my spot meter on the area just over the top of the birds use this reading to set my camera and shoot the scene as metered this would place that color as a medium or mid-tone orange, rendering my subject as a dark featureless silhouette.
The correct exposure for a sunrise or sunset silhouette can be a very subjective thing; generally as long as your subject is rendered a dark featureless shade the background can be as light or as dark as you choose.
This series of tips as well as many others are posted on my facebook page
Good luck and good light