Combine the dried boletes and soy sauce in an airtight container such as a mason jar. You can also just remove a little soy sauce from the bottle, smash up the dried mushrooms and stuff them right in the original container with the soy, that will work just fine too.
Store the jar in the refrigerator and allow it to age for a few months before using, then use anyway you would regular soy sauce
Notes
The high level of salt in the soy will preserve the mushrooms near indefinitely, 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. A way to get around this is just to put the whole shebang in the blender once the mushrooms are fully rehydrated, this way they will sink to the bottom of the container. You could also just strain the soy off after aging and discard the mushrooms.