Instructions
Speak with Gotz, the builder in town, and ask him to build you a Mushroom House. You're able to build it out of grass, wood, stone or gold. If you use a strong material, the house is less likely to fall down or become damaged.
Enter the Mushroom House when it is finished being built. Walk up to one of the six pallets. Select the pallet to put a small log on top of it. This is where your mushroom will grow. If you have more logs, put them on the other pallets.
Get mushroom spores. Collect Toadstools and Matsutake mushrooms from the wild during the fall season. Since these mushrooms are wild, you must process them to get spores. If you want Shiitake spores, go to Vesta's shop and buy them for 3,000 gold per bag.
Process Toadstool and Matsutake mushrooms if you have a Maker Barn. The Maker Barn is another building Gotz can erect for you. Inside, put the mushrooms into the seed maker to extract the spores.
Go into the Mushroom House and place the desired spores onto one of the pallets. Cover all the pallets with spores to get the best harvest.
Water the mushrooms every day. If you don't, they are very likely to shrivel and die.
Wait for the mushrooms to grow and mature. This takes a long time: 35 days for Shiitake, 42 days for Toadstool and 59 days for Matsutake. Let the mushrooms get as big as possible before picking them. Larger mushrooms are worth more money.