Stacking hair is done to even up the tip ends to build bodies, heads, wings, legs and tails of flies. Commercial hair stackers are commonly used, but if you don't have one, your fingers can be used for the same purpose. There are many good stackers on the market. I prefer the Renzetti stacker because it has both a small and large cylinder. Both stacking methods are shown below.
Commercial Stackers
Cut a clump of deer hair and use a fine-toothed comb to remove the underfur. Then place the hair, tips first, into the stacker and hold it slightly at an angle as you tamp them down. If you stack the hair vertically, the hairs might fall to the side unevenly and will have to be sorted out. To finish, hold the stacker horizontally and pull out the lined-up hair.
Finger Stacking
If you don't have a commercial stacker, you can use your fingers to even up the hairs. To begin, tightly grab the longer tip ends of hair with your gingers and pull them out. Then, replace them in the stack so the tips are even with the original bunch. Repeat this process until the hairs are all even.
Note: A strong thread is necessary to avoid breakage as you spin the hair. A nylon thread, Kelvar and Nymo will prevent this from happening. Also, other stackers such as Brassie and Anvil are reliable choices.