Oh we have lots of dried shiitake at my house! Yum!
-Let them soak overnight, put a plate ontop of them or something to make sure they're submerged.
-Cut off the ends of the stems as they're usually too hard to chew well, even after soaked.
-Cook them with rice to give your rice a mushroomy flavor, and serve with soy sauce.
-Cook them in a soup (like miso soup, or any other really) as they give a really nice mushroom flavor! Actually, cook them with anything you want to have a mushroom flavor. They need to be boiled a good bit to become tender!
My mom usually cooks them with meat (obviously not exactly an option anyone here would choose to pursue) but I bet boiling seitan with them would turn out great! Also, boiling them and then chopping them up and mixing them into your seitan for a nice mushroomy flavored seitan would likely be good.