Clean the mushrooms very well, then weigh them in grams. If you want to eat the ferment straight out of the bag, steam them first, then weigh.
Multiply the weight of the mushrooms by anywhere from .02-.05% to get the weight of salt needed. For example, 1000 grams of mushrooms you would use 20-50 grams of salt.
Combine the mushrooms with the salt, then quickly vacuum seal the bag. Use a bag larger than you'll need to allow extra space at the top for resealing the bag once or twice during the fermentation process.
Over the course of a few days, you'll notice the bag puff and inflate from carbon dioxide, snip off the corner to release the air and "burp" it, then carefully re-seal without using the vacuum setting.
Allow the mushrooms to ferment in a cool dark place with a stable temperature (don't ferment them outside) like a pantry. Taste them after 5 days. I like them aged anywhere from 5-10 days depending on what I'm doing. Remember over-fermenting can make them mushy.
When you're pleased with the flavor, transfer them to a mason jar, or another non-reactive container, along with every bit of juice from the bag. The mushrooms will last for a month, or longer if you are careful to keep them completely covered by their juices.
Notes
You'll get decent flavor from mushrooms fermenting them anywhere from 7-14 days. If you forget about them in the vacuum bag, don't worry, the ferment is stable, and forgiving.