How To Ripen a Tomato:
The easiest way to ripen a tomato is by controlling the temperature. Warmer temperatures speed up the ripening process and cooler temperatures slow it down.
To help a tomato reach its full flavour potential, place it in a warm spot in the kitchen. You don’t even need direct sunlight, any warm spot will do. If you want to accelerate this process even more, place tomatoes in a paper bag to concentrate the ripening gas the tomatoes produce naturally and keep it in a warm place.
If you need to prolong the life of a tomato, place it in a cooler area of the kitchen and you’ll get a few more days to use it. Although you want somewhere cool, it’s best to not put the tomatoes in the fridge, as too cold of a temperature will turn the flesh of a tomato mushy and mealy. If you must place it in the fridge, do so after it has ripened and make sure you allow the cold tomato to come to room temperature before using to improve the flavour.