It's pretty frustrating when you have a taste for guacamole, but only have rock-hard avocados on hand. Don't fret. Follow these instructions and those hard avocados will be soft in no time.
Place the hard avocados in a paper bag. The bag will trap the ethylene gas the avocados produce that causes them to ripen.
Add a banana or apple to the paper bag. These fruits will produce additional ethylene gas and help the avocados ripen even faster.
Keep the bag closed for 1 to 3 days and store in a dark, cool location. The avocado should be ripe after 3 days, but start checking its softness after the first 24 hours.