Grain spawn
One jar of grain, sterilised properly and colonised end to end, is the unit that everything after this is measured in. Get this stage right and the bulk stage is mostly carrying things.
Grain
The nutrition the mycelium eats through before it moves into the bulk bed.
Why: Grain is chosen for the number of separate growing points per jar rather than for anything mystical. Rye, oats and wild bird seed all work, and growers argue about the order.
Improvised: Whole oats from a supermarket are the cheapest reliable start and the easiest to prepare well.
Jars or spawn bags
Holds the grain through sterilising and colonising.
Why: Jars survive a pressure cooker indefinitely and let you see every side of what is happening. Bags hold more, need an injection port and a filter patch, and are thrown away afterwards.
Improvised: Wide mouth canning jars with a drilled and ported lid are the reusable option, bags are the single use one.
Pressure cooker
Holds the grain above boiling point for long enough to sterilise it.
Why: Boiling water pasteurises, it does not sterilise, and grain is nutritious enough that anything surviving in it will win. This is the single piece of kit that decides whether the stage works.
Improvised: A stockpot is only ever a pasteuriser. That is enough for a bulk substrate later, and not enough here.
Impulse sealer (optional)
Closes a spawn bag with a clean heat seal.
Why: Only relevant if you are using bags. A folded and clipped bag works, it is just slower and easier to knock open.
Improvised: Fold the bag twice and clamp it with a bulldog clip, or use jars and skip this entirely.
Gypsum (optional)
A spoonful through the grain keeps the kernels from gluing themselves together.
Why: It is a quality of life improvement rather than a requirement: separate grains shake apart properly, and shaking is how you speed a jar up.
Improvised: Skipping it costs you a slightly stickier jar and nothing else.