Step 1 Install a 6 Mil Plastic Moisture Barrier
Overlap the seams as recommended by the manufacturer, then roll out the foam underlayment. Other subfloors need only foam underlayment. Some planks come with the underlayment already attached. If your planks do, you won't need to put more down. Choose the wall you'll start flooring against and roll a single strip of underlayment along it. Roll additional underlayment as you install the planks.