Verified answer

If you're working with cardboard spawn it's only semi sterile and your best bet is to put as much cardboard spawn with the grains as you can, however I fear you will only end up with contaminated grain. You're are better off starting on agar then moving the colonized agar to sterilized cereal grain then to wheat straw.

there is plenty of info on cultivation of mushrooms here: www.ediblemushroom.net