Usually when it comes to growing mushrooms, the most important thing that you will need to begin with is the mushroom spawn itself - this is what the mushrooms grow from, so it is an essential ingredient. Without it, it would be like trying to grow an apple tree without the apple pip - its never going to work. Usually the mushroom spawn will be especially created by a mycologist, and will normally consist of some kind of grain that has been impregnated with mushroom mycelium (mycelium is what the mushrooms come from).
You can create your own mushroom spawn several ways, but what most people don't realise is that you can actually grow your own mushrooms from old stem butts, enabling you to re-use old mushrooms and start your own cycle of mushroom cultivation.
All that you need to do is to cut off a small section of the mushroom stem butt, no bigger then an inch long. It is important to try and use the bottom of the stem, with the bulbous part of the stem intact (this part is usually removed prior to preparing the mushroom for cooking and for packaging before being sold in stores). However if you have already grown some of your own mushrooms from some kind of spawn then you will have no problem getting hold of the healthiest variety of stem butts. You can still try using the stem from shop-brought mushrooms, but you may not have as much success.
It is important stating that not all mushrooms have this stem growth capability, and there are only a few varieties that are known to be able to re-spawn from their stems. These include Oyster mushrooms (Pleutrous Ostreatus), Parasol mushrooms, Morels, Prince, and many more.
All you need to do is remove a section of the stem butts, and get some corrugated cardboard. Soak the cardboard and then try to peel it in half, so that the paper is as thin as possible. Next, lay the carboard on a surface and place some of the stem butts on top. Try and leave around 6 inches of cardboard per stem butt. Cover the stems with the rest of the corrugated cardboard and then re-soak for a few minutes. Place the folded pieces of cardboard into an old box - cardboard, wood, or anything else that will keep the box moist, and then place it in a shady position in your garden before covering it with leaves.
What will happen is that the mycelium will start to grow through the pieces of cardboard, from the stem butts, using the materials in the cardboard as a food (mushrooms naturally grow on wood and so cardboard is ideal to use and is easier to break down). After a few months you will have your own cardboard spawn, and you can either use this to grow mushrooms on more cardboard, or you can mix this cardboard spawn with straw, creating an outdoor bed or mushroom patch, or you can try and transfer the spawn to other substrates.