Spread them out on the trays but don't worry too much about them touching.
Set the dehydrator to 135F (57c).
Dry for 6-10 hours, rotating trays if you think about it. Move marshmallows around on the trays as they become drier to avoid them sticking together.
They're ready when they crumble when you try to squish them between your fingers and they don't "give" when squeezed
Notes
If you have larger marshmallows, you can cut them into smaller pieces with kitchen shears. Dust with a bit of homemade powdered sugar to keep them from sticking together. Or dry them whole for 24 hours.The marshmallows will continue to dry a bit as they cool, so do your final test on fully-cooled marshmallows. You can't over-dry them.