Measure the wall sheathing — both the width and the height — using a tape measure.
Cut the felt paper with a utility knife to fit the measurements you took in Step 1. For multiple sheets, add 6 inches to the paper width for overlapping at the felt paper seams.
Set the first sheet of felt paper against the wall sheathing, beginning at the right side of a wall. Secure the paper to the wall, using roofing nails. Place the first nail in the top leftmost corner of the sheet approximately 2 inches from the edges of the paper, and use a hammer to drive it through the felt paper and into the sheathing material. Place additional nails in the paper in a row extending downward with each new nail placed every 6 inches.
Move to the right 6 inches and place another row of nails to secure the paper. Continue with a new row every 6 inches until you've covered the entire sheet of paper, stopping 2 inches from the lower right hand corner of the sheet.
Place the next sheet above the first with an overlap of 6 inches at the seam. Repeat the mounting process with the roofing nails. Continue until you reach the top of the wall, and then place the next row of sheets to the left of the first with an additional 6-inch overlap. Attach the felt row by row until you reach the end of the wall on the left.
Cut the felt so that you can create 4-inch flaps for window the openings. Place the flaps around the window, laying them from the outside of the opening inward. Slit the flaps at the corners of the windows so they lie flat in place.