The process is really simple, the only thing you have to take care is to sterilize everything during the process :your hands, the mold, all the material you will use. you can order a kit which can be shipped from every and grow you own form:

The kit comes as a small bag of Mushroom Materials, which have been dried so they don't need refrigeration. This dehydration process also means that the product can be packaged and internationally shipped.

Preparation

Open the Bag, Add water and Flour, mix it well, close the bag but let the air enter trough a filter on bag.

wait for 5 Days : the bag become white ( mycellium is growing) .

After 5 Days

sterilize your mold, fill it with the material, press it a little bit , cover it with plastic wrap, punch some small hole, and put it in the dark.

5 Days more...

Wait for 5 ot 6 Days , remove it carefully from the mold, let it dry one or two days.

preheat oven to ~100°C then Bake it for 30 Min Note : you can bypass the oven and let the mushroom grow outside the mycellium shape but I think that it will be more fragile.

Links&Inspiration

Please find Hereby the best Guide I found from ecovativedesign.