Place grapes onto the dehydrator screens. Try to not let them touch.
Close the dehydrator and turn onto high.
Dehydrate on high for 24 hours, rotating the trays every 2 hours or so. Don't fret if you don't rotate the trays at night while you're sleeping!
Turn the dehydrator down to 135 F and continue drying for a total of 3 days, rotating the trays every 2 hours or so. I like to move the grapes around a bit as they are drying to keep them from sticking to the screens.
Notes
You can fit about a pound of grapes per tray in the Nesco Snackmaster Pro. Do not overcrowd the grapes.