If your tomatoes have bruises, remover them.
Wash and then dry the tomatoes.
Take your tomatoes and slice thin. The thinner the better because they'll dehydrate faster.
Set your dehydrator to ~145F.
If your dehydrator has non-stick trays, use them. If not, line the trays with parchment paper.
Put the tomato slices on the trays and ensure none of them are overlapping.
Dehydrate overnight or for 16+ hours.
The next day, take them out and allow to cool for 10 minutes. If they still feel soft/pliable, they need to keep dehydrating. If they're brittle and snap, they're done.
Put into a pestle and mortar or spice grinder and grind into a fine powder.
Store in a storage bag or glass container and use as needed.