The underlayment has a plastic strip on one end that goes along the wall and up the wall that will be covered by the baseboard, and the other end has a strip of adhesive tape. As you unroll each roll, the clear plastic end lies on top of the end with adhesive strip and two white pieces of foam will be butted up against each other.
Tip: Make sure the two white sides are as close as they can be and lay one over the other and tack it down. That is how you will know if it's on upside down -- if the tape strip is down, it is backwards. It is very important that the underlayment run the same direction as the flooring.