Almost everyone will be familiar with the small built-in flashguns
found on every type of compact camera. If you’re not, just turn the
camera round and take a picture of your own face. Once your eyes have
stopped watering, that bright green blob you’re now trying to blink out
of your field of vision is the exact shape of your flashgun, so just
look on the front of your camera for a clear window of the same shape.
It may take several tries to get this right.