Make sure your tahini is mixed well so that the oil is not separated. It should be smooth and uniform.
Add the tahini and lemon juice to a food processor or blender and process for at least 1 minute. It should begin to thicken and form a bit of a paste. You may need to stop and scrape the sides with a baking spatula and then continue.
Add the olive oil, garlic, salt and half the garbanzo beans (rinsed and drained). Process for a full minute or longer.
Add the rest of the beans and process until smooth.