If you use dried mushrooms, you first need to rehydrate them before cooking. To do this, soak them in warm water, wine or stock. Twenty minutes or so should do it.
When the mushrooms are soft, lift them out of the liquid with a slotted spoon.
Don't toss the liquid! It's full of flavour - but also sediment. Filter the liquid through a coffee filter or a clean, damp piece of kitchen paper. Add it to your recipe - to flavour to soups, stews or sauces. Or freeze it to use later.