How to ripen green tomatoes
Tomatoes need warmth,
not light, to ripen so there’s no need to put them on a sunny windowsill. While the warmth from the sun does encourage fruit to ripen, you may end up with overly dry fruit and this is especially true for fruit that’s almost completely green.
Instead, place your tomatoes in a paper bag and position the bag in a warm spot. The paper bag acts as a miniature greenhouse, offering warmth and trapping ethylene. Like many fruits, tomatoes emit ethylene gas as they ripen, which accelerates the ripening process. Bananas are prolific ethylene producers, so place one in the bag with your tomatoes to speed up the process.
If you have several tomato plants with unripe fruit on them, uproot the whole plant and hang it upside down in a warm but shaded spot, like a shed.