Legendary mycologist Paul Stamets in his book Mycelium Running popularized the idea of stem butts as a low tech cultivation method.
This is how i do it. When collecting Pleurotus sp. for example I make sure to pick them with the stem butt intact. I get a few pieces of corrugated cardboard that is brown and without print on it. I put them in a pot of water and bring them to the boil. I drain and let cool with the lid on. when they are cool i peel off one side of the skin to expose the corrugations and place the stem butts on the cardboard. Roll it up and place it in a ziplock bag with a label and date. I put this in a cool dry place and after a few weeks it will be evident if they are growing.
when the cardboard is showing healthy growth its time to expand. there are a few possiblities:
More boiled cardboard
boiled fresh wood chips or sawdust for wood decomposers
sugar cane mulch or straw
Oyster mushrooms will grow on an amazing array of organic waste.
To continue my example i will grow the oysters onto a shopping bag full of boiled wood chips until they start to fruit or the mycelium is well colonized. From here i can start an out door bed without the need to pasteurize the substrate. I will clear a shady spot in the garden, lay out some cardboard and wet it with the hose, then about 5 to 10 cm of fresh wood chips, again wet them well with the hose before spreading the bag full of ‘spawn’ on and covering with more wood chips and watering with the hose. if its a hot or dry time of year i can then mulch the bed with clean straw, slashings, lawn clippings or sugarcane mulch. After a few months to a year mushrooms should start to appear. Be sure the mushrooms are in the right species!
This technique will not guarantee success but its a simple low cost way to start experimenting with mushrooms.