We just spread them whole (but minus stalks) on trays or racks, and either put them in the bottom oven of the Rayburn (with the door open) or leave them on top of the Rayburn at nights. Don't let them get too hot or they slowly cook and go all inky. You just want to dry them slowly.
In the absence of a Rayburn (or similar) you could use a food dehydrator, or maybe put them on trays in the airing cupboard?
Then break them up into kilner jars.
Keeps for years. Easily long enough to tide you over a couple of poor mushroom seasons.
You don't need to use much in cooking as the flavour is more intense than fresh mushroom.
(Parasol mushrooms are particularly good for drying, if you're lucky enough to find any. Giant puffballs are excellent too).