Mash the avocados, lime juice and salt in a molcajete or medium bowl. Mix in the onion, cilantro, tomatoes, and garlic. Stir in the cumin.
Serve immediately.
Notes
To keep the guacamole from turning brown, put it in an airtight container with half an inch of water on top. That should keep it from browning for about three days! Be sure to store the container of guacamole in the refrigerator. Drain the water off when you're ready to use the guac.