Preparing Growth Substrate

Once your spawn is thriving you can choose and prepare a substrate, and finally prepare a mold. The Growbio GIY material uses hemp fiber as a substrate, which works great. You can order hemp fiber bedding for animals to use but it looks a bit pricey to me. My preference is to find readily available materials to use. The best material I've found is shredded cardboard, which requires a heavy duty paper shredder I have access to.
Some other materials I've tried, or want to try:

Whatever substrate you choose, it must be soaked for several hours to absorb water and soften up. Overnight is okay unless you're using something like paper that would start to disintegrate.

Drain the water so the material is at field capacity moisture.

Add the material to the canning jars. It can be packed in loosely or packed tight.

Pasteurize in the pressure cooker as with the spawning grain.

Once it has been pasteurized for 45-60 minutes let it cool down, either in the jars or in a container you will use to mix it with the mycelium spawn. Make sure to sterilize the mixing container before adding the substrate. For mixing I use something like a large foil pan or large tupperware, depending on how much substrate plus mycelium spawn I plan to end up with.

Now you are ready to mold and grow!