In a half-gallon or larger container combine 5 cups of water and 5 ounces of hydrogen peroxide and set aside.
Squeeze the unopened spawn bag to break the spawn into small pieces — the smaller the better. This may take a little effort.
Cut the spawn bag open across the top, just below the seal.
Pour the coffee grounds and pellet mixture into the bag with the spawn.
Fold or roll down the top of the bag a couple of inches, hold it closed firmly (taking care not to poke the filter patch), and shake until mixed — the more thoroughly mixed the better!
Open the bag and pour in the water + peroxide.
Use a clean, peroxide-dampened paper towel to wipe the upper part of the inside of the bag clean.
Fold the top inch of the bag over and tape it securely, sealing it as well as possible to prevent dust or insects from getting in. Don’t tape over the filter patch! Make sure you leave air space in the top of the bag for circulation.
Let it sit for an hour or so to let the pellets fully expand.
Massage it gently to shape the substrate into a loaf. Squeeze it upward a bit so it does not press too tightly against the sides of the bag.
Your mycelium will begin colonizing the substrate right away. You should see growth within a day or so!