Selecting the correct hackle feather is paramount to creating a good hackle fiber tail on a dry fly. The feathers with the very best fibers are generally located on the very outer edges of a hackle cape. You want to select a feather that has the longest, stiffest and straightest fibers before plucking it free from the skin.
Disregard the lower webby fibers, and look more toward the middle of the feather. Use your thumb and forefinger to preen the fibers down the stem until you find a group of 8 or10 that look good. After preening them perpendicular to the stem, grab these fibers by their tips and strip them free by pulling away the stem. The idea is to keep the fiber tips as aligned as possible.
With the fibers free, measure them to form a tail about a hook shank in length and then transfer that measurement rearward to the start of the hook bend. Lay the fibers at an angle against the near side of the hook and begin making thread wraps to secure them. Thread torque will help to carry the fibers to the top of the hook shank where they can be bound down further.