How did Voldemort learn horcruxes? A master, tell me.

When Voldemort was tom riddle in the sixth grade, he read about horcruxes and their making methods in the Encyclopedia of Dark Magic in the library (killing people, making their souls unstable, and then splitting their souls and attaching them to an object), but he was eager to know whether humans could make more than one horcruxes to ensure eternal life.

So he asked Lagerhorn, a potions professor, and got a positive answer (Lagerhorn was shocked, but he didn't expect Tom to be so ambitious and cruel), so he started making horcruxes.

During the summer vacation from grade five to grade six, he made his first horcrux: the pendant box of Slytherin with the money of his uncle morphine Gunter. Since then, he has made seven horcruxes, one of which (Harry Potter) was made unconsciously.