When it comes to ripening any fruit, ethylene is your friend. This gaseous plant hormone is produced by all fruits as they ripen, and will prompt unripe tomatoes into turning red. Place the fruit in a paper bag, cardboard box or wooden drawer with a ripe banana or apple, ensuring you leave out any damaged or diseased fruit which will rot the lot. Check them regularly and you should find the tomatoes are ripe within a couple of weeks.
Fun Tip: Place half your immature green tomatoes in a box or bag for ripening. Then with the other half, make some wonderful food using green tomato recipes. There are so many great ones available like; fried green tomatoes (always delicious and easy to make), soups, relish, jams, pie, and breads.