[header] How to ripen an avocado [title] Place the unripe avocado into a brown paper bag. [step] The bag is used to trap the ethylene gas, which ripens the avocado. Make sure there aren't any holes in it! [substeps] The paper bag is just a trap. 
If you can devise some other mechanism that will trap in the air in the same way, great! That can be used, too. Your grandma might tell you to store it in the flour bin, but you may have to settle for the empty mcdonald's bag.