If you are following a pattern, make sure that you leave at least the amount of thread recommended in the instructions. Sometimes these thread tails will be used to add other embellishment or findings to your beadwork.
If you are adding a new thread, make sure that you leave at least three (3) inches of thread as a tail to securely and properly weave it into the beadwork. If you do not leave enough thread, you will not be able to maneuver your needle through the beadwork and knot it securely.