Lay the underlay starting at and parallel to the eaves maintaining a consistent tautness.
How to felt and batten a hip roof.
When the first length is complete lay the next layer of felt on top of the first so that it overlaps by at least 100mm 4 inches horizontal overlaps where one roll ends and the next one is started should be at least 150mm 6 inches.
Fold the excess flap of roofing felt over the hip and staple the felt in place along the hip 4 to 5 inches from the edge of the hip.
Work along the roof taking out any excess slack in the felt but do not pull it tight.