1. You need subject with strong outline and it's easy to recognise just by its shape.
2. A strong light source coming from the back of your subject.
3. Make sure silhouette subject does not overlap with each other.
4. Expose for the second brightest part of the scene. For example in photo A, i took a exposure reading from the area to the left of the sun and set my aperture and shutter-speed accordingly.
Photo A.