Clean the subfloor with a broom or vacuum cleaner. The floor should be dry prior to installing the underlayment.
Unroll the underlayment along the longest wall of the room. Cut the underlayment at the wall using the utility knife. Unroll another length of underlayment immediately next to the first piece.
Butt the two pieces together. Apply tape along the entire seam for maximum moisture and heat loss protection. Continue unrolling and taping the underlayment until the entire floor is covered. Install the flooring material according to manufacturer's instructions.