How do I dehydrate Ganoderma Applanatum?
As the generic simple answer that works for most mushrooms:

Cut them in cubes of 1 cm³ (0.061 inch³)
Put them on parchment paper in your oven at 110°C (230°F)
for 1 hour (½ Shí).

doesn't seem to work on this particular mushroom, you'll need special (read: expensive) equipment to be able to dry these before being able to grind them to powder and that is:

Freeze drying
Vacuum drying

Sorry!  :-(
Note: Or bring a few six packs to someone owning this equipment already and drink the beers while waiting for the process to finish