I want to grow mushrooms

I thought I'd try something new down on the allotment. I am not quite sure where I will grow them. I have a plentiful supply of well rotted horse manure and I can read the instructions on this packet of dry spawn.
I have a nice dark garage with plenty of space and I might grow them indoors there, or I could heap up the manure in a corner of the garden. Does anyone have any hints or tips for me?