Summarize:

For the first part of the process, you will need to scorch the tomatoes at a temperature of 425 degrees Fahrenheit (218 degrees Celsius). Preheat the oven to that temperature first.  Meanwhile, prepare two baking sheets by lining them with nonstick aluminum foil or parchment paper. You could also spray the baking sheets with nonstick cooking spray if you do not want to use foil or parchment paper, but note that foil and parchment paper will make it easier for you to clean up later on. Use a rimmed baking sheet so that the excess juices and liquid produced during the process will be contained instead of dripping off into your oven. The tomatoes will need to be washed, dried, cored, and cut. Removing the seeds is an option, as well.  Note that you should not skin the tomatoes yet. Rinse the tomatoes under cool, running water and pat them dry with clean paper towels. Cut a cone shape out of the top stem end of each tomato to remove the core. Use a small paring knife to do so. Cut the tomatoes into decent sizes. Cherry tomatoes should be cut in half, plum or roma tomatoes should be cut into halves or quarters, and larger tomatoes should be cut into 1/4-inch (6.35 mm) slices. You can remove the seeds, if desired, but the seeds and pulp contain a lot of flavor, so many prefer to leave them in. If you decide to remove the seeds, scoop out the seeds with your finger or with a kitchen spoon and leave as much of the pulp behind as possible. Arrange the tomatoes on your prepared baking sheet with the cut-side facing up. Position them so that each piece is about 1/2 inch (1.25 cm) apart from the others. Do not stack the tomatoes or let them touch. Doing so could result in tomatoes that do not dehydrate evenly, so some may end up drying out or burning while others remain too moist to use. Popular seasonings for dehydrated tomatoes include salt, ground black pepper, herbs, garlic powder, and onion powder. Sprinkle the tomatoes with your seasoning of choice generously, or according to taste.  If you use herbs, go for options like oregano, parsley, and thyme. Both dried herbs and fresh herbs will work. You could also scatter fresh minced or chopped garlic over the tomatoes instead of using garlic powder. Drizzle a fine stream of olive oil over the tomatoes, coating them evenly.  This oil enhances the flavor of the tomatoes and prevents them from cooking poorly. If using store-bought olive oil in its original container, place your thumb over the spout as you pour so that you can control the speed and size of the stream more easily. Use your hands or tongs to flip the tomatoes over so that the skin-side faces up. This is important since you will be scorching the tomatoes before you dehydrate them fully. By exposing the skin side to the direct heat, you prevent the tomato pulp from becoming too burnt too quickly. Place your tomatoes in the preheated oven and let them sit there for 30 minutes. When ready, the skins will show signs of some wrinkling and some browning. Remove the tomatoes from the oven and pour off the excess liquid that has begun to leak out from the tomatoes. Remove the skins by pinching them in between tongs and peeling them off.  You can drain the liquid by tilting the pan and letting it run off into a bowl, or you could also suction the liquid away using a turkey baster. As soon as you pull the tomatoes out of the oven, you should reduce the temperature to 300 degrees Fahrenheit (149 degrees Celsius). Do not finish roasting the tomatoes at 425 degrees Fahrenheit (218 degrees Celsius). Return the tomatoes to the oven and cook for another 3 to 4 hours. The finished tomatoes should look visibly dry with a few dark edges.  Flip the tomato pieces cut-side up after the first hour of cooking. Drain or siphon away the excess juice every 30 minutes. Remove the tomatoes from the oven and let them cool at room temperature. When ready, you can store them in an airtight container or plastic bag, which can be frozen for up to three months. Alternatively, place the tomatoes in a bowl and cove them with extra-virgin olive oil. Wrap the entire bowl in plastic wrap and refrigerate the tomatoes for up to 2 months.
Preheat the oven. Prepare the tomatoes. Place the tomatoes onto baking sheets. Season the tomatoes as desired. Drizzle with oil. Flip the tomatoes over. Blister the tomatoes. Drain and skin. Roast the tomatoes. Store.