I use mycelial division. (basically, take the white fuzzy bits the mushrooms grow out of, break it up into pieces, then distribute it into sterilized, damp media, and let it grow.)
I got a very small tupperware container of such material from my older sister last year. I have been keeping it going all last summer, and all winter. Harvested edible mushrooms several times from the newly cultured media.
Scriver has a commercially purchased grow kit. It is comprised of a mostly decorative cardboard house, a bag filled with spawn-infused medium, and a care instruction pamphlet.
The bag itself is the fancy bit of that kit. It appears to have a fancy strip of breathable material around it, which allows sterile gas exchange with the outside environment, while keeping germs, and foreign fungal spores out.
Typically, the bags that these grow kits use have small perforations in the bag, such that when fruit body growth is initiated, they are able to poke through/tear the bag all on their own.
Now, about the general concept:
Mushroom growth is moderated by carbon dioxide levels.
Very high CO2 levels, and the mycelium starts to die from suffocation
Fairly high CO2 levels, and the mycelium "Runs"-- eg, it spreads into a fuzzy mass, and colonizes the medium quickly.
Moderate CO2 levels (with exposure to light), and the mushrooms pin, then make long and gangly stalks with small caps (which are prone to abort)
Low CO2 levels, the mushrooms stop pinning, but mushroom pins that formed will produce high quality mushrooms.
The bag Scriver got in his kit is, again, the fancy part of the kit. It allows ideal gas exchange at each stage of the mushroom's development, but is sadly NOT re-usable.
Simplified:
1) Sterilize your medium before inoculation.
2) Sterilize your growth chamber before putting medium inside.
3) Inoculate your medium under as close to sterile conditions as you can.
4) Allow only low levels of gas exchange during the initial incubation of the culture, but DO allow it to breathe. This promotes quality mycelial mass formation.
5) Increase ventilation of the medium, so that pinning starts, but do not allow it to be fully open to air. Permit exposure to light. (indoor lighting is sufficient)
6) Once pinning starts, allow full ventilation, but keep mushrooms moist. Continue exposure to light.
7) Allow mushrooms to mature, then harvest.
8] If your chamber permits, restrict ventilation again to restart mycelial run behavior, then restart. Repeat until medium exhausted.
9) If your chamber does not permit this (such as the plastic bag that scriver has), or your media has become exhausted, break up the mycelial mass into small, "shooter marble" sized bits, and distribute it in fresh, sterilized medium, in a freshly cleaned (or brand new) container. (eg, if you are using bags, use a brand new bag. If you are using a sealable tupperware tub, move to a freshly cleaned and prepared tub.)
10) do the process over and over and over again.