There are two options.
First, less common but still conceivable in some cases, requires grinding or cutting the mushrooms into smaller chunks or even powder. Then they can be used directly as a spice just thrown into the mixture while cooking. For some reason, that is not a very popular method in Poland but it can be employed especially if you don’t want to use much of the product or just intend on adding a distinct hint of the flavour.
Most recipes though require soaking the mushrooms in hot water. That rehydrates them, extends their volume and makes them more suitable for further processing. If you have a very high-quality product containing big chunky pieces of mushrooms, they can sometimes even imitate the fresh mass after soaking. However, I wouldn’t be that hopeful and you definitely won’t mirror the same effect. Anyways, the soaked mushrooms can be added directly to the dish only if the recipe requires still a long time of the simmering process. If not, or more precisely if it takes less than 30-minutes to cook the whole dish after mushroom addition, then they have to be fried on a pan (or simmered separately) prior to their implementation. The whole point is to make them as soft and succulent as you could require from cooked fresh mushrooms. The additional benefit of the soaking method is that the liquid remaining after soaking has also a very strong colour and wild mushroom taste. You can then use it at the end of the cooking as an additional product to regulate the exact flavour by adding part of it basically like stock.