Combine the dried boletes and soy sauce in small pot, bring to a simmer, then cool and store in an airtight container such as a pint mason jar.
Store the jar in the refrigerator and allow it to age for a few at least 30 days before using. Remove the mushrooms, squeezing out any soy into a bowl to add back to the jar. Use the mushroom soy anyway you would regular soy sauce.
Notes
You may be tempted to leave the mushrooms in the jar for longer. The high level of salt in the soy will preserve the mushrooms near indefinitely at room temperature. But, if you pour off some of the soy sauce to cook with it and expose the mushrooms to air borne bacteria, they may begin to mold.