Here's an excerpt form Experimental Brewing, a book by Drew Beechum and me that will be released next Nov.
To start the process, you'll want to visit fungi.com a few weeks ahead of time and get some mushroom spawn. There are a number of varieties for your choosing, but Mike recommends Oyster or Old Man's Beard for beginning mycologists because of their rapid growth and culinary usefulness.
The basic concept is: dry the malt, kill the various organisms (including lactobacillus) found on the malt. Allow the mushroom spores to consume the malt and its nutrients before exposing it to the sun and encourage fruit (mushrooms) to form. All the while, you want to prevent exposure to dust to avoid contamination of the fragile colony.
After a few weeks work (really, mostly just spend ignoring the whole thing) you'll be rewarded with a big pile of glorious mushrooms ready for chow time.
[Technique: Growing Mushrooms]
• • Foil lined cookie sheet
• • Malt to fill the sheet
• • Mushroom spawn (1/10th of the malt, by weight)
• • Hydrogen Peroxide (1% of the malt weight)
• • Paper Grocery Bag
• • 2 Plastic Totes (Clear/Translucent)
1. 1) Fill the cookie sheet with malt and dry in a 450F oven for 30 minutes.
2. 2) Mix the malt and hydrogen peroxide.
3. 3) Break up the mushroom spawn into tiny pieces. Each piece will serve as a colony.
4. 4) Mix the spawn and malt and place in the grocery bag. Fold the bag top over and secure it with a light weight. (You want oxygen exchange, but no dust)
5. 5) Place the bag in a tote or crate and store in a dark location that is about 65F
6. 6) When the mushroom colony looks like a big white cloudy mass, it's time to encourage the colony to fruit. Move the bag to a warmer spot (75F) with visible sunlight. Cut X's into the bag to provide spawning locations. Cover the bag with the other plastic tote to retain humidity for improved yield.
7. 7) Harvest and enjoy!
Mike also advises would be mushroom farmers to read up on the hobby and become aware just how far away from sterilization the typical homebrewer's practice of sanitation actually is. The whole process is fascinating with many varieties and variations of fungi and techniques.