Cut avocados lengthwise, around pit. Separate the halves and cut lengthwise again along the pit half, allowing the pit to come out easily. Cut the non-pit half in half as well, and peel off the skin from all four quarters.
Place the peeled avocado into a large bowl and squeeze half a lime over them, plus 1 tsp of lemon juice (optional, helps to keep from browning). Let sit while you cut onions, garlic, cilantro, and tomato.
With a potato masher, mash the avocados to the consistency you desire (smoother or chunkier). Then fold in the onion, garlic, tomato, cilantro, salt, cumin, and cayenne. Let sit at room temperature for at least 10 minutes. Taste. Add more salt or lime juice depending on your taste and/or avocado ripeness.