You can do whatever you want but this technique is pretty fun and efficient.
> Cut it in half, one end to another. We cut in half to get a flat surface so that the onion doesn't roll while we cut it. We cut it end to end to make the next steps easier (you'll see)
> Cut out one of the pointy ends from each half. But keep the root end
> Peel the outermost layer. Throw it if it's rotten inside. But if just the outer layer is discolored, probably fine.
> Make hanging slices. Slice with the tip of the knife towards the root and the blade towards the center. But don't slice all the way to the root so that it stays in one piece. This makes the next step easy
> Slice the other way. Instead of slicing straight down with the knife, slice near the tip of the knife and pull towards yourself. This way nothing sticks to the back of the knife, nothing rolls to the floor, and everything stays in a big pile you can pick up easily with one hand. This is also very space efficient on the cutting board, which is nice if you're cutting many things, or have a small board