Avocados are one of the most temperamental fruits. Rock hard and unripe one minute but mushy and brown the next, they have their own mind on how quickly they choose to ripen.
Avocados ripen because of the ethylene gas they produce. If you want your avocados to ripen faster, trap the ethylene gas by storing the fruit in a closed paper bag.
Additionally, when storing avocados in a refrigerator, make sure that there aren't any other ethylene-producing fruits like apples or bananas sitting next to them.
Unripe avocados have a bright to dark green hue with only spots of black on them. When the skin darkens to an almost-black shade, you'll know that the avocado is ripe.
When an avocado is sliced, the exposure to air causes it to oxidize. To slow the browning, either tightly wrap the avocado in cling film, or brush it with lemon juice or oil.