Turn the flash on and set it to manual. This is normally achieved by cycling through the MODE button on most flashes. If you’re not sure about this bit, check the information in your flash manual.
Adjust the power.
Now you can adjust the power by pressing the arrow keys. 1/1 is full power, and each click down will decrease the power.
Set the camera
Place the flash on the hotshoe of the camera and switch the camera on. Next, turn the camera to manual. Set your shutter speed to 1/125 and then pick your aperture and ISO. If you’re going a step further than this and are using radio triggers or a TTL lead, use these instead of placing the flash on the hot shoe of the camera.