You will need :
Slice off both ends and remove the peel without damaging the pulp.
After you've rinsed it, slice the onion in half.
Lay the onion flat on the board, and cut the bottom.
Then slice parallel to the lines on the onion.
Cut against the lines if you want to create circles.
To dice the onion, do not cut off the bottom.
Orient the onion such that the bottom is facing upwards.
Cut the onion, horizontally, 3 or 4 times, without cutting the bottom.
Give it a quarter turn and continue vertically, still without cutting the bottom.
Finally, cut the onion vertically while holding it with your fingers.