Twist the bag shut. What you have now is a kind of terrarium. The mushrooms-to-be, however, need to breathe, so over the twist goes a small collar cut from a tube of PVC [pictured], and it is rubber-banded there. Then fluff open the "collar" of the bag. Now the bag can breathe through the PVC tube. But to keep other spores and things out while mushrooms are developing, stuff the PVC tube with a thumb's length of that synthetic fill that goes into pillows. Now the bag can breathe but nothing can get in.
If you are able to get mycelium (by mail order; it takes a scientist like Smith to grow it correctly), all the rest is very low-tech.
I will keep you posted as to what's happening with my mushroom kit. Nothing is more exciting than something growing.