To ripen an avocado quickly, place it in a paper bag with a banana or an apple, as these fruits release ethylene gas that speeds up the ripening process. Fold the top of the bag to trap the ethylene gas, and leave it at room temperature for 2-3 days. Check the avocado periodically, and once it reaches your desired ripeness, store it in the refrigerator to slow down further ripening. This is the only method I recommend to ripen an avocado!