If you have time, buy very firm, even hard, avocados a few days ahead of time and let them ripen on your kitchen counter. Ripe avocados are very dark green in color and slightly soft to the touch. If you buy one that is too soft, it's often bruised and brown inside.
The citric acid is optional, but it provides an added tangy flavor and helps the guacamole keep its bright green color longer. You can vary the amounts of any of the ingredients to your taste — or use just avocado, garlic, lime juice, and salt for a quick basic version.
Slice the avocado in half lengthwise and open it. Remove the pit (see technique). Using a tablespoon, scoop the meat of the avocado out of the skin, and put it in a medium bowl. Mash the avocado with a fork until it is about halfway smooth.