The Horcrux is a device of exceeding complexity for which the consequence of failure is a swift demise. The prize for success, however, is immortality. For the reader that feeleth that he canth meet the challenge, takest a moment to consider this course of action before thou hast destroyed thyself.
The Horcrux is a nigh-impregnable fortress for the portion of the soul contained within, violable only by a small handful of methods. With the proper protection, the Horcrux will never be harmed, and its creator will be eternal.
Some may choose less effective, unproven means by which to secure an eternal life. It canth only be said that they will never survive for as long as one with a Horcrux. Manethon of Sebennytos lasted for three hundred years, a feat unlikely to be bested by one without such a secure hold on immortality as Manethon's Horcrux.
It is recommended that something important be chosen as the Horcrux, but the object must also be innocuous enough to pass for a typical magical object. It doth no good if a Horcrux proclaimeth itself to all who would wish to do harm to it, after all.
Should death ever befall the creator of a Horcrux, knowest that an indeterminable length of pain and suffering awaits thee before thou canst enter thine eternal rest. Remembrest that thou hast committed what can only be described as a violation against nature, a blasphemy of life itself. Thy life, as always, is in thy capable hands.
A word of advice: Be sure of heart and swift of mind if the Horcrux is what thou dost seek to create. Many have died in attempts, and several that succeeded were driven mad by the effort. This is a warning, designed to ensure that the Darkest Art be kept secret and hidden. Were this knowledge to come into the hands of the commons, the line between life and death would be forevermore blurred, the results too catastrophic to ponder.