Hey everyone
The way to score with a downward header, is to cross the ball from the wing / edge of box
When one of your approching players is about 2-3 metres away from the ball,
you need to double tap
Holding the first tap slightly longer as this produces the power (just like a normal shot)
and by tapping
again this will activate the downward header.
Now this is all about timing!
It took me a few attempts, but by playing in beginner mode and with the sliders adjusted in my favour I was able to attack down the the wings and consistently cross balls in until I got the timing spot on
Enjoy!