My mother showed me how to ripen a fruit when I was a child and I've been doing it that way ever since.
It is easy but takes a little time.
Pan through the pictures to see how to tell an overly ripe avocado from a hard avocado -- the good ones have a black skin, a little shine but not too much, and when you touch them, they give a little. But be careful... squeezing them causes bruises if you are not going to use it right away. Just be gentle. And don't let the bag boy pack them with the cans because they are hard -- that causes bruising, too. I often ask them to bag them separately.
All you have to do is put your unripe fruit into the brown paper bag and roll down the top to close the bag.
Set it aside somewhere in the kitchen and leave it at room temperature (not in the refrigerator). On top of the fridge is a perfect place. Just let it sit over night (for probably two nights -- depending on the individual fruit -- possibly three for a really hard green one)
This will naturally speed up the ripening process so that in a day or two -- as you check it -- it will ripen. It may not change the color of the skin but the fruit will be ripening in that brown paper bag. I don't know why this works, I just know it does.
It does seem to ripen even faster if placed on a window sill, or on a counter where the sunlight can reach it through the window.
I've seen many people ripen things by simply placing the fruit (especially tomatoes) on the window sill but if you place one or two in a brown paper bag this will speed up that process by a day or two.
I often buy 4, 5, or even six hard avocados at a time (depending on the price) and ripen them at different speeds. One or two I put in the brown paper bag. One or two I simply place on the counter top. One or two I leave in the fridge. That way I can have one (or two LOL) each day for a few days in a row.
If you forget them (LOL) you'll have squishy tasteless avocados... eat them only if you want to!!