Very low heat and / or air movement
In the UK a dehydrator or oven on lowest setting with the door cracked (better yet if its a fan oven)
For use, some things are great dried (jerky, apple rings etc.)
To rehydrate generally soak in water then cook in liquid. A nice crumble can be made of apple rings ad blackberry leather for example.
Plenty of threads on here about dehydrating and I have a load more if you need guidance
Red