You've already received some good advice, but I thought I'd add a few more thoughts.
I would recommend shooting with your camera in manual mode. This will give you more consistent exposures with the flash. It's often a good idea to take a few test shots before turning the flash on to check your in camera exposure. Then turn on the flash. There are two ways to set your flash for fill flash.
- You can use ETTL with your flash set at approximately -1 to -2. Check the in camera thumbnail and the histogram after shooting a couple of shots and then adjust accordingly. This is the best way to set your flash if your subject distance is unknown or variable.
- The second way is to use manual flash exposure. This will give more consistent results than when using ETTL. This works best when your subject distance is known and consistent. The flash should be set to equal approximately 1/2 the distance to your subject. Check the distance gauge on your lens to get the distance to the subject, then check the setting on the flash press by pressing the shutter halfway, you will see a little black bar underneath the distance scale on the back of the flash. Adjust the flash power to get the reading you desire.
- To maximize flash output, keep your shutter speeds at or lower than the high speed sync setting of the camera 1/250 for many cameras, 1/300 for the Mark III and IV. You will have to vary aperture and or ISO to keep the shutter speed down.
- If you need a higher shutter speed then you will need to shoot in high speed sync. See article link below
- The Better Beamer will give you approximately double your flash output. You don't need to change anything when shooting ETTL, but when shooting in manual flash mode set your flash to equal 1/4 the distance.
- Practice with stuffed animals of different tones and different settings to get a feel of it.
The above settings are for fill flash. If your subject is in the shade and background is in the sun, or similar light scenario then you will be using Flash as Main Light.
When shooting with Flash as Main Light
- Your camera exposure will be for the background. Take a few test shots to check your background exposure. (approximately 0 to -1 EC)
- The flash exposure will be for the subject. For ETTL - your flash will be ~ 0 FEC - though this is variable depending on the tonality of the subject and the ambient light.
- For Manual Flash the distance on the Flash should equal the distance to the subject (1/2 the distance when using the BB). Depending on the amount of ambient light on the subject, you may need to decrease the Flash Power.
For more reading try the following articles in the NS archives
High Speed Sync
Better Beamer
Another very useful resource for learning flash is
Strobist. While the site is devoted to off camera flash, the principles and techniques discussed are very informative.