- Know your flash's power
- Know what you want the image to look like
As with any image, you need to have an idea of how you want the image to look before you shoot it. Do you want there to be shadows behind the subject? What about light shadows angled on the face to help define features? Do you want an even light across the entire image? Questions like these must be answered in your mind before using fill flash. These answers will determine if you use an on-camera flash, an off camera flash, a diffuser on your flash, what angle the flash should be at, and how close you put the flash to your subject.
Once you have the answers to these questions you will know what flash to use, if a diffuser is needed, and where to place the flash. From that point its a simple matter of pressing the shutter button to finish your image.

