We make our own prunes... delicious!
Start with slightly under-ripe fruit (of which you have a surfeit). Not totally under-ripe – not hard and green – but not soft and sugared either. They’re going to sweeten up a lot as they dry, so you need them to start out on the tart side.
Wash the fruit and cut each plum in half. Remove the pits then spread the fruit out on dehydrator trays and load up the machine. Dehydrate between 105-115º (F) for roughly 24 hours, or until prunes are shriveled and darkened.
Alternately, place them in your oven on baking trays with the oven set at the lowest possible temp... it'll take maybe 12 hours that way... still looking for very dark and very shriveled...