Need some help with "waterproof" paneling?

My guess: install strip on the first piece, and run a screwdriver down the narrow slot on side two with just enough twist to open it slightly--don't over-do it. It should hold itself open that extra bit long enough for you to get in piece two. Add piece two at an angle away from the first (looking horizontal plane), and use it like a pry bar to work your way in. I don't know how you'll get the adhesive in after-the-fact.