Making a Horcrux is a truly EVIL ACT.  To make a Horcrux you need to kill people- Killing people-murdering them! When you kill a person you will be able to make 1 Horcrux. The full process is unknown the other thing you have to do is an act of unspeakable EVIL that an ordinary witch or wizard can't do. When killing a person- 

The specific processes involved are known to involve a spell and a very horrible act. To split one's soul, one must also commit the most supreme act of evil—murder—and then encase a portion of their fractured soul into a chosen object with the spell. The detached soul fragment will always remain as it was when it was divided; for example, Tom Riddle's diary portrayed Tom Marvolo Riddle as a teenager, and the eyes inside Salazar Slytherin's locket resembled Voldemort's eyes as they were when he looked more human.

Though a Horcrux can be made from anything (including living creatures), Lord Voldemort chose to use objects of great significance or importance. The process makes the part of the soul remaining in the witch or wizard unstable. If the maker's physical body is later destroyed, he or she will live on in the non-corporeal form, although there are methods of regaining physical form. However, according to Horace Slughorn, few would want to live in such a form and death would be preferable.

It is stated at one point that Voldemort had already "pushed his soul to the limit" in creating his seven Horcruxes. This implies a finite number of Horcruxes any one person may create before the process becomes too dangerous to attempt again. Though this limit is never explicitly stated, the number seems to set solidly at seven intentional Horcruxes, and creating seven Horcruxes in addition to the person's own body renders the soul unstable and liable to break off when the person whose soul it is to commit murder. Dumbledore explicitly stated that Voldemort's soul had become so unstable that it finally "broke apart" when Voldemort tried to murder Harry for the first time.