Okay – on to the onion, which, you have no doubt observed, rolls around easily on your cutting board. This is not an optimal situation in the presence of a sharp knife, so the first step is to cut off the top of the onion to make it flat. Note that I cut off the end opposite from the root, where more of those sulfur compounds are stored. Some chefs cut off both ends, but, as you will see, this makes it easier for the onion to fall apart while being diced. Your choice.
The objective, after all, is to produce neat and uniform pieces that look good in your food and, more importantly, cook evenly because they are all the same size. (This is one of the reasons I told you to throw away that chopping gizmo. Its final product is a mix of irregular pieces.)