When you kill someone, does your soul immediately split, or is there a process? [duplicate]

Question

I've read the Harry Potter books many, many, many, many times, and this has never exactly been clear to me. Do you need to do something in addition to murdering someone to make a Horcrux, or does it automatically happen?

Answer 1

There is a process that needs to be gone through.

It involves a spell of some kind, more than just killing a person, but exactly what is required has never been elaborated upon.

This is explicitly stated in the books, the very first time we encounter the concept of a Horcrux:

"By an act of evil - the supreme act of evil. By committing murder. Killing rips the soul apart. The wizard intent upon creating a Horcrux would use the damage to his advantage: He would encase the torn portion -"
"Encase? But how - ?"
"There is a spell, do not ask me, I don’t know!" said Slughorn, shaking his head like an old elephant bothered by mosquitoes.

_{-- HP and the Half-Blood Prince (emphasis mine)}

Answer 2

JKR: Yeah, but I would imagine that other people, you know, other people are going to have tried. I think it would be naive not to think that people have been trying for a long time, and thought they succeeded and hadn't, or else, or else you know maim themselves or kill themselves in the attempt. It's such a dangerous thing to do.

SU: Oh. Evil thing. You know, just ...

JKR: Yeah.

MA: What is the process? Do you - Is there a spell? Is there a - What do you have to do?

JKR: I see it as a series of things you would have to do. So you would have to perform a spell. But you would also - I don't even know if I want to say it out loud, I know that sounds funny. But I did really think it through. There are two things that I think are too horrible, actually, to go into detail about. One of them is how Pettigrew brought Voldemort back into a rudimentary body. 'Cause I told my editor what I thought happened there, and she looked as though she was gonna vomit. And then - and the other thing is, how you make a Horcrux. And I don't even like - I don't know. Will it be in the Encyclopedia? I don't know if I can bring myself to, ummm ... I don't know.

Accio Quote!

Also from canon - at the Burrow before the trio set out:

Hermione rummaged for a moment and then extracted from the pile a large volume, bound in faded, black leather. She looked a little nauseated and held it gingerly as if it were something recently dead.

'This is the one that gives explicit instructions on how to make a Horcrux. Secrets of the Darkest Art - it's a horrible book, really awful, full of evil magic.'

...

'And the more I've read about them,' said Hermione, 'the more horrible they seem, and the less I can believe that he actually made six.'

Harry Potter and the Deathly Hallows - pp.88-9 - Bloomsbury - Chapter 6, The Ghoul in Pyjamas