The top 2 spawn grains of choice are rye berries (high quality, but difficult to come by at reasonable prices) and wild bird seed (cheap and easy to find, works just as well as rye). Most cultivators that have access to rye prefer it because of it's uniformity. I will detail how to prep wild bird seed here because of it's availability. If you have access to rye, I suggest visiting Myco-Tek, or the Shroomery and viewing a rye grain tek there.
You will need the following supplies
Pressure Cooker with a 15psi setting
Quart jars with filter lids
Wild bird seed (Pennington's is popular, but any wild bird seed that is low in corn will do)(1 20lb bag does 30 ~ 40 quart jars)
Wire mesh strainer
Large cookie sheet with paper towels
Step 1: Start by measuring out the amount of birdseed you need to fill your jars. You'll want about 2 cups per jar so that there will space to shake the grains, and perform grain to grain transfers. If you have no intention of doing a grain to grain transfer, 3 cups will work as well. Note that when doing calculations for spawn ratios, 2 jars = 1 quart since each jar is only going to be half full, or 2 jars = 1.5 quarts if you use 3 cups of birdseed.
Step 2: Place the bird seed into your strainer and rinse well with lukewarm water. Rinse until the water runs clean.
Step 3: Place birdseed into your stock pot and cover 2 inches over with hot water. Let sit 12 to 24 hours.
Step 4: After the sit time, turn the burner on high, and heat the water until boiling. Stir occasionally, once every 4 to 5 minutes is fine. When the water reaches boiling, immediately pour the birdseed into your strainer. Don't allow it to boil for more than a minute.
Step 5: Allow the birdseed to dry in the strainer, tossing it occasionally. Drying should take between 30 and 90 minutes depending on how often you toss the birdseed.
Step 6: Load 2 to 3 cups of birdseed into each jar. Place the lid on the jar, tighten down the ring fully. Cover the top half of each jar in foil if your filter requires it. Load jars into the pressure cooker and put on the lid (add water to the pressure cooker as per the instructions that came with it). Allow the cooker to vent steam for 10 minutes before putting on the weight.
Step 7: Heat for 120 minutes once pressure has been achieved. When time is up, remove from burner and allow to cool over night. Once cool, give each jar a little shake to separate the kernels.
You can use a spore syringe (1cc injection), a liquid culture syringe (1cc injection), an agar wedge, or grain to grain transfer to inoculate your jars. An agar wedge can be placed into the jar with the aid of a still air box, glove box, or flowhood. A grain to grain inoculation can be done in the same manner by taking a fully colonized jar, shaking it up, then adding some of the colonized grains to your fresh jars. Grain to grain transfer is a wonderful method of turning 1 colonized jar into 10! Once fully colonized, spawn to the bulk substrate of your choice.