Slice the tomatoes thinly and place the tomato slices on baking sheets or dehydrator trays, spreading the tomatoes out so that they are not touching or on top of each other.
If using an oven, bake the tomatoes at the lowest temperature, using the oven fan if you have one. If using a dehydrator, dry on a low setting. either way, continue to dry the tomatoes until they are dry and brittle. You want the tomatoes to be brittle because that's a good sign that you have thoroughly removed the moisture from the tomatoes. If they are still flexible, they likely have a tiny bit of moisture still. It's a good idea to flip the tomatoes partway through the process, especially if using the oven.
Once the tomatoes are completely dry, you can leave them as is, storing them in an airtight container or in a jar with olive oil (see instructions below). You my want to first slice them into strips for using on salads.
If you want to make tomato powder, you will want to grind the tomatoes into a powder using a blender or food processor. I find it easier to first cut the tomato slices into smaller pieces and then grind them into a powder in the blender. If the powder isn't fine enough, you can switch to a coffee grinder to finish grinding the powder.