This is how to make porcini powder, but it works with any dried mushroom. Morels and black trumpets are other great options. It assumes you are starting with fresh mushrooms.
If you are working with boletes like porcini, slice them in half. Any that are marginal, as in slightly wormy, should be dried. Ones with no worm holes should be eaten fresh. Remove any of the "sponge" that is yellow and reserve.
Slice the mushrooms and arrange them on a rack for the dehydrator, along with the sponge if you have some. Dehydrate on low heat, ideally about 110°F, until they are crispy dry.
Put all this into a spice grinder and grind to a powder. You might have to return larger bits for re-grinding.
Store in a glass jar, ideally with a silica packet, in the pantry. Will keep for years.