Summarize the following:
Avocados oxidize quickly. Cutting them at the absolute last moment ensures that they stay fresh and green. Start by cutting an onion in half. Halve again until you have one quarter. Dice the onion with a sharp knife. Set aside in a small bowl. If you want onion with a less acidic bite, wash your diced onion under cold water in a colander and drain. Water will help remove some of the sulfuric acid in onions (the same sulfuric acid that makes you cry when you cut an onion).

Summary:
Start by dicing your onion.