Growing the Psilocybe cubensis on a substrate of straw is the best choice if you aim for growing at a somewhat larger scale, for it is easier to produce than a rye substrate. The straw delivers a better feeding ground for the Psilocybe cubensis than rye does; the mushrooms seem to grow larger and more numerous on straw.
First, you have to prepare the spawn. You begin with inoculating a jar with rye, just the same as you would make the jars for the cultivation of mushrooms on only rye. When this jar with rye is fully colonized we are not going to use it as substrate. However, we are going to use this jar with mycelium on rye to inoculate a bag with straw. When you distribute colonized rye throughout straw, the mycelium will develop further throughout the straw. When you use mycelium this way to inoculate another substrate it's called spawning. In this method the colonized rye will serve as spawn for straw. For a standard filterbag with straw you need about 2 jars of spawn.
First the straw needs to be chopped to a maximum of 5 cm. If the straw is too long the mycelium will have difficulties with colonizing the straw. When the straw has been chopped to the right length, it is time to soak it in water. This can be done by putting the straw in a large plastic bag and fill this with water. Watch out with this, it can be become easily a mess. Let this stand for at least a few hours, although some cultivators prefer to let it soak for even 2-3 days. After the soaking the straw has to be drained very thoroughly. Finally fill a filterbag with the soaked/drained straw. Then the filterbag with straw has to be sterilized (15 psi, 3 hours) or pasteurized (60-70 °C, 2 hours). Either way, don't close the filterbag with an impulse sealer yet, the spawn has still to be distributed throughout the bag at a later moment.
When the sterilized or pasteurized straw is completely cooled off, it's time to divide the spawn over the bag of straw. Before you inoculate the straw with the spawn, carefully shake the spawn in the jar so the chunks of mycelium break apart. The straw can now be inoculated with the spawn. Open the filter bag with the straw and pour the mycelium in the bag. Close the bag immediately with a good sealing. It's important to evenly divide the spawn over the straw, to avoid the substrate from not colonizing completely. Especially with a mass of moist straw in a filterbag this can prove to be be a troublesome job.
Finally place the full filterbag in the incubationroom. Again, let time do its work and after 2 to 4 weeks, the straw will be fully colonized with mycelium. Now it's time to prepare the final phase. Take a large clean box, spread out the colonized straw with a height of 7-10 cm and put a casing layer on it. Thoroughly cover the box with plastic foil. Place the box in the incubationroom again, and give the mycelium time to form a strong network through the casing-layer. After a couple of days, when the mycelium is clearly visible through the casing-layer, it can be exposed to fruiting conditions. Under normal conditions the first pinheads will show up after 7 -10 days.