What you will need:
A sharp knife
A cutting board
Grab an onion. A simple, yellow onion will suffice.
Chop the non-rooted end off.
Chop it in half.
Peel it down to the shiny-whites.
Lay it flat and put your palm on it. (Fingers & thumb up!)
Put a good slice in it, parallel to the cutting board. Watch those fingers!
And another, a little further up--still parallel to the cutting board.
Keep slicing it, all the way to the top.
Turn the onion 90 degrees so the rooted side is away from you.
Start slicing it--perpendicular to the cutting board now.
Slice it all the way across
Rotate the onion 90 degree again, back the way it was when you started.
Slice it, perpendicular to the cutting board, but parallel to the last slices.
Like a grid.
So much math involved in this.
Keep going, you're almost there.
Discard the rooted-ness.
Now you try!
Let me know how it goes.