My favourite knife to use is a victorionox serrated knife.

Cut off the root. Leaving as much onion there as possible as this will be very useful later!

Cut off top

Cut in half

Peel

You need to know which side is the root and have this to the left. This will hold the onion together when chopping. If your left handed have the root to the right!

Start to slice but not all the way through

Should need 3-6 "slices" depending on size of onion

Cut most of the way but not through root

Cut once horizontally, again not through root.

Slice downwards

The same width as the slices

Till you get to the root

Place this part flat and cut into 2-3 pieces

Large dice onion! :-)

Let me know how you get on! You can change the size of the slice to dictate the size of the dice! :-)