Unroll about two feet of felt paper from the roll and lay it at the top of the roof valley, with the remaining roll facing downward towards the roof eave.
Unroll the rest of the length of felt paper to the eave and cut it, leaving about a foot of extra material.
Center the entire length of felt paper over the roof valley (overlapping other felt paper strips, previously installed on the roof face) and tuck it to match the valley contour.
Fasten the top of the felt paper strip to the roof sheathing (plywood) using a roofing stapler and roofing staples.
Fasten the rest of the felt paper strip, working your way from the top to the bottom. Space the staples about 16-inches on center, unless otherwise required by local building code or felt paper specifications.
Trip the bottom end (at the eave) of the felt paper strip to match the shape of the eave; do the same at the top of the strip.