Bill is right about adding the ripples, a third way, the way I've always done it, is right before the resin sets, which may take 8 hours, more or less, depending on heat and humidity, is to take a screwdriver and prod it. Keep prodding until the ripples set.
I know this sounds time consuming, but you don't have to stand there for 8 hours, more or less, waiting. 
Come back and check on it. I guess the only advantage to doing it that way is that the ripples are a part of the "water" and not a facade.
