Trim the stem, keep the root intact: Trim the stem end first. The root end holds the layers together, so the onion doesn’t fall apart as you cut. Don’t cut the root off, only the stem.
Halve the onion through the root: Cut the onion in half and peel off the skin, any papery parts of the onion, and the first outer layer if it feels tough.
Horizontal cuts first: Stabilize the onion by putting it cut-side down on the cutting board and hold the top of the onion steady with your fingertips. Make several horizontal cuts toward the root (slicing nearly all the way through but not quite all the way through to keep the root intact so the onion doesn’t fall apart as you dice).
Vertical cuts toward the root: Then make parallel vertical cuts all the way toward the root but not through it.
Slice across the cuts: Cut downward, creating perfectly uniform diced pieces. You’ll be dicing the onion by making a series of vertical cuts perpendicular to the ones you just made.
Sizing: Slice the onion in whatever thickness you desire (small, medium, or large) for what you’re using the onion for.