You will need: an onion; a chopping board; and a sharp knife.
Chop in half, vertically
Chop the head (NOT root end) off
Peel the onion; you will be left with a naked onion that looks like this.
Slice the onion horizontally, but be careful to leave the root end in tact!
Your onion should now look beautiful, like this one.
Now cut horizontally across your onion, stopping 1/2cm from the roots (which can now be discarded).
Now you have a beautifully diced (1/2) onion. Go forth and enjoy.