I don't drain the tomatoes of their seeds and liquid as I want as much flavor as I can get. This of course is optional.
Place on screened trays for your dehydrator. If you don't have screens you can place them on parchment paper. You can spray a very fine mist of cooking spray or oil on your trays to keep the tomatoes from sticking, however, I rarely do this.
Dehydrate at 150F or whatever your highest setting is. Remember, we are going to dry these to a crisp!
After about 3-4 hours open your dehydrator and loosen the slices and flip them over. This will keep them from solidly sticking to the screens or parchment.
Continue to dehydrate until slices are completely dehydrated, dry and crisp.
Remove and place in blender. (I use a Magic Bullet for this and it works great).
Blend to a fine powder and store in a jar or zip lock bag.