Cut your avocados in half, remove the pit and score the meat of the avocado. Then with a spoon, dig out the meat of the avocado, scraping along the inside of the avocado casing so you get all the dark green stuff (the dark green stuff is the most nutritious.
Next you want to juice your limes (or lemons) over the cubes of avocado and start smashing them with your potato masher. This does two things:
It allows the juice from your citrus to coat and be absorbed my the avocado meat more evenly, preventing oxidization
It breaks up the avocado so you start to get a smoother consistency
Personally I like my Guac a bit chunky, so I leave some of the avocado bits kind of still in tact. Also, unless you want a super smooth, no chunk guacamole, you want to leave some of the pieces in tact before you add the rest of your ingredients. As you mix in the rest of your ingredients, your avocados will continue to break down and become less chunky.
Next add your diced onions, minced garlic, chopped cilantro and diced tomatoes and mix it all together with a spoon. Add salt and pepper, mix and taste it!