Preheat your oven to 70℃. Take a tray out and line it with baking paper.
Cut each tomato in half over the length and then in four quarters per half over the length as well (so each tomato in 8 parts).
Put all the tomato pieces skin down on the baking paper, spreading them evenly over the tray. Make sure they don't touch each other.
Put the tray into the oven for about 10 hours. You can turn the tray around every few hours if you like, because most ovens don't heat evenly in the front and the back.
Your tomatoes are ready to use when they're gummy, but not completely dried out. I sometimes take part of the tomatoes out already while leaving other ones in a bit longer, because despite the turning around of the tray, they're not always all ready at the exact same moment.