I am pouring a concrete slab in 2 stages. How do you install that felt-like material in between the two slabs?

I know that a heavy felt material is used to join two slabs of concrete. I think it is for expansion and contraction. How do you install this material when you are pouring the slab? Is it best to install it when the first of the two slabs is poured, or is better to pour up against it with the second slab? How much does it cost and where can I buy it?