Prep the subfloor by countersinking all screw heads that stick out above the flooring surface. In addition, scrape away any wood glue that may have squeezed up between the subfloor seams. Vacuum the subfloor thoroughly to remove any bits of sawdust and debris.
Roll out the felt paper, starting at one side of the longest wall, and cut the other end of the paper with a sharp utility knife. The felt paper should fit within 1/4-inch of the opposite wall. Position the long side of the felt paper within 1/4-inch of the back wall, as well.
Use a 1/4-inch hammer stapler to attach the felt paper to the subfloor. Insert one staple approximately every foot. If any staples stick up, pull them out and insert new staples.
Cut the second row of felt paper and butt the seams together tightly but do not overlap them. Staple the second row as you did the first one. Continue laying the felt paper in rows until you cover the entire subfloor.