Fill flash is a technique that illuminates the darker areas of the image especially the subject. When the subject is under exposed it is good to use fill flash to bring exposure but not too much. For working on a portrait when using fill flash

make sure that you focus on keeping it natural and subtle. Another thing is placing the subject in an angling position with the sun, making use of a reflector to ramp up the glow on the subjects face. For landscape photography make use of the

fill flash to sparkle the foreground, also you may need to use gels for the color temperature adjustment. Lastly just focus on a natural look.