You do need to soak the skin in warm water before you apply it to the back of a bow. If you don't you will have problems smoothing the skin and getting any air bubbles out from under the skin.
You will have about 30 minutes of working time with the skin wet and almost none with it dry. You need this extra time to center the pattern and work excess glue and air bubbles out from under the skin.
You can put the skin on dry with contact cement but you better get it perfectly positioned the first try because you won't have a second chance.