Create a white solid.
Keyframe the opacity.
You might also play with the blending mode of the layer.
You also might want to play with an adjustment layer over the footage and mess with things like blur and levels or curves.
Without any specific reference it's hard to know exactly what you're after. A flash of light at death is not the same between films.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.