You can use a fan or an oven or whatever, but if you want it done right and you want it done quick, there's only one way: Dehydrate.
It goes like this:
1. Buy a food dehydrator 2. Put the mushrooms in, you may need extra trays. 3. Dehydrate at max temp until cracker dry. (cracker dry means they break upon any attempt to bend them. None of them should be bendable) Max temp is usually about 160-165. If yours goes up to 175, go ahead and crank it. I would only start worrying about the mushrooms cooking (read:cooking, not potency loss) past 180 or so. Might affect color but it's not commonly found to affect potency.
This takes anywhere from 4-24 hours depending on the size and amount of mushrooms. Don't stop drying until you can confirm they're fully dry or they'll oxidize hard and fast in the absence of air movement.
A few things:
-Heat does not kill potency. This question comes up all the time, but it's more or less a misunderstanding of oxidation. The active components can withstand temps of up to 300F without significant decomposition. -The faster they are dried, the less oxidation occurs due to moisture and air exposure, which is the primary cause of potency loss, not heat. - A dehydrator is a lot cheaper than a wasted effort growing.