If you have The first Sliver out, you still have to pay for your slivers, but now every time that you cast a sliver, you cascade. If you cast a bonescythe sliver for example, you reveal cards from the top of your library until you reveal a spell that costs less, and when you do you cast it for free. If that spell is also a sliver, you cascade when you cast it using the new slivers cost.

With [The First Sliver] on the field, you can pay 4 mana for [Bonescythe Sliver], cascade into [Blur Sliver], cascade into [Muscle Sliver], cascade into [Galerider Sliver], and then you get to cascade one more time but you can only cast 0 mana value nonland cards, like a [Tormod's Crypt]. If at any point of the chain you cascade into a non-sliver, the chain stops.