To make a Horcrux, certainly murder can't be the only requirement. It is obvious a proper spell is needed for completing a Horcrux. Voldemort could have made a thousand Horcruxes, as he had already murdered a thousand people. But, how on earth did Harry become a Horcrux, with Voldemort murdering his mom and not using the Horcrux-making spell? Certainly Avada Kedavra is not the spell used for making a Horcrux.
J.K. Rowling addresses whether or not Harry is a proper Horcrux in Pottercast with The Leaky Cauldron (citation below) and reveals that Harry is not a Horcrux. Read on:
"Here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul."
"I do think that the strict definition of "Horcrux," once I write the [Harry Potter Encyclopedia], will have to be given, and that the definition will be that a receptacle is prepared by Dark Magic to become the receptacle of a fragmented piece of soul, and that that piece of soul was deliberately detached from the master soul to act as a future safeguard, or anchor, to life, and a safeguard against death." JK Rowling - Pottercast 12.23.07 - The Leaky Cauldron
No, Harry was not an official Horcrux. There is a spell used to complete the process, which JKR has not yet revealed.
From Half-Blood Prince:
A Horcrux is the word used for an object in which a person has concealed part of their soul."
"I don't quite understand how that works, though, sir," said Riddle. His voice was carefully controlled, but Harry could sense his excitement.
"Well, you split your soul, you see," said Slughorn, "and hide part of it in an object outside the body. Then, even if one's body is attacked or destroyed, one cannot die, for part of the soul remains earthbound and undamaged. But of course, existence in such a form ..." Slughorn's face crumpled and Harry found himself remembering words he had heard nearly two years before: "I was ripped from my body, I was less than spirit, less than the meanest ghost. . . but still, I was alive." "... few would want it, Tom, very few. Death would be preferable."
But Riddle's hunger was now apparent; his expression was greedy, he could no longer hide his longing.
"How do you split your soul?"
"Well," said Slughorn uncomfortably, "you must understand that the soul is supposed to remain intact and whole. Splitting it is an act of violation, it is against nature."
"But how do you do it?"
"By an act of evil — the supreme act of evil. By commiting 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. " Do I look as though I have tried it — do I look like a killer?"
"No, sir, of course not," said Riddle quickly. "I'm sorry ... I didn't mean to offend . . ."
Half-Blood Prince - chapter 23 - Horcruxes - US Hardcover
Although the spell is unknown, there exists a spell that is used to create horcrux, intentionally of course.
As explained here,
Creating a horcrux requires one to commit a murder, which, as "the supreme act of evil, (...) rips the soul apart." After the murder, a spell is cast to infuse part of the ripped soul into an object, which becomes the horcrux. The spell can be found in the book Secrets of the Darkest Art.
Voldemort had already created five horcruxes before going to Godric's Hollow to kill Harry, (and I suspect he was going to use Harry's death for making his last horcrux, although I have no canonical evidence to support my assumption). This evil act had made his soul highly unstable. [Ref: Page no 114, Deathly Hallows]
“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. It warns in this book how unstable you make the rest of your soul by ripping it, and that’s just by making one Horcrux!”
When Voldemort attempted to kill Harry, his killing curse rebounded off the protection spell and killed him physically. But due to the instability of his soul, a part of his soul was blown off during his death and sought out the nearest living thing, which incidentally was Harry. This is how Harry became Voldemort's sixth Horcrux, his accidental and unintentional Horcrux.
You ask "with voldy murdering his mom and not using the horcrux making spell?". The answer was provided by Dumbledore (emphasis mine):
“Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort’s soul was blasted apart from the whole, and latched itself onto the only living soul left in that collapsed building. ...” [ Deathly Hallows, Dumbledore finally explaining to Snape the full story as Harry is observing from Snape's memory ]
So, there were two events that happened to make Harry a "Horcrux" (quotes are there on purpose - see Slytheincess' answer):
Avada Kedavra, when bouncing off of Lily's protective magic around Harry, hit Voldemort himself instead, and ripped off a part of his soul.
Once his body was dead/gone/destroyed from Avada Kedavra, that ripped off soul piece tried to find a new host, and Harry was the only host available.
For making a horcrux,certainly murder is not the only rule.
Murder is not the only rule, but it is a necessary step in order to create a horcrux.
It is obvious, we need a proper spell for making one.Voldy would have made a thousand horcruxes, as he already murdered a thousand people.
He only wanted 7 because he believed 7 to be the most powerful magical number.
But, how on earth Harry became one, with voldy murdering his mom and not using the horcrux making spell? Certainly Avada kedavara is not the spell for making horcrux.
That's explained in @ykombinator's answer.
My two cents: just as Voldemort handpicked very exclusive/rare objects for his horcruxes, he handpicked his victims as well. James or Lily were two meaningless people on his way to power. He could dispose of them. Harry, on the other hand, was the one he marked as an equal after hearing about the profecy. So Harry, of course, would have been the last and most important victim of the set.
What I don't really know is whether Voldemort had already picked an object to make a horcrux with when he went to kill Harry. I guess the spell is to force the part of soul to "store" itself in a specific physical container. In absence of the spell (since Voldemort's body was gone, so he wasn't able to perform it), the part of unstable soul found a life-form in baby Harry and stayed there, something Tom Riddle did not intend or foresee.
When Voldemort went to Godric's Hollow, he had already created 5 horcruxes. He was timing all his horcrux creation to significant killings and important objects to encase part of his soul. Harry was the biggest threat to Voldemort created by the prophecy. It must have seemed a very important killing for Voldemort. He was so intent on making Horcruxes and to evade death in all possible manner that he had forgotten basic rules of old magic.
Therefore, when Voldemort went to Godric's Hollow, it can be assumed that he went prepared to create a Horcrux with the killing of Harry Potter. What was the supposed object cannot be said with certainity.
We do not know the procedure of making a horcrux as it has not been discussed or described. It is possible that the wizard intent on making a horcrux has to cast the horcrux creating spell, followed by killing to rip the soul apart and then some other spell to encase the soul fragment.
But as the curse to kill harry potter rebounded, it split voldemort's soul as planned. But Voldemort's body was gone. He could no longer encase the soul fragment to complete horcrux formation. Soul has a property to live in a body. Only a encasing spell can put the soul outside the body in a physical object. Voldemort's soul which was left unstable latched itself to the only living thing it could find, Harry himself.
This was the reason that Harry contained the soul but was not posessed by it, rather it was becoming a part of him and providing him qualities like being a parselmouth.
I completely support ykombinator's answer with this additional thought:
Voldemort was an extremely skilled and powerful wizard. He had spent years to hone his skills. That means he came prepared. Since Voldemort was mortified by death, it's reasonable to assume that making horcruxes wasn't his only preventive action. I think he protected himself with the most powerful artifacts and spells available, as well.
So there might have been a spell to transfer his soul into a new body should he be killed. Or maybe he unleashed the horcrux spell as a last-second attempt to do something when he realized he was dying.
All in all, it doesn't feel right that he would die without any struggle after all these efforts to stay alive.
Yeah! Even I feel the same. Voldemort was so obsessed with making the last horcrux that he was actually waiting for the opportunity but the prophecy must have interrupted his mind and he might have thought to finish it before moving to make the next one. So, he might have jumped to Godric's Hollow to kill 'em all. But, since several things must have crossed his mind, which might have made him weaker and the spell he cast was so weak that Lily's magical love destroyed voldemort.
As said by Slytherincess, JKR did approach this question:
Here is the thing: for convenience, I had Dumbledore say to Harry, "You were the Horcrux he never meant to make," but I think, by definition, a Horcrux has to be made intentionally. So because Voldemort never went through the grotesque process that I imagine creates a Horcrux with Harry, (SU: Mm-hm.) it was just that he had destabilized his soul so much that it split when he was hit by the backfiring curse. And so this part of it flies off, and attaches to the only living thing in the room. A part of it flees in the very-close-to-death limbo state that Voldemort then goes on and exists in. I suppose it's very close to being a Horcrux, but Harry did not become an evil object. He didn't have curses upon him that the other Horcruxes had. He himself was not contaminated by carrying this bit of parasitic soul.
I like to assume that since when Voldemort's soul split an additional time from being so unstable, and the piece attaches to Harry (the closest living thing it could survive off of), it kept Harry from being killed with the killing curse. After all, you can't kill a Horcrux with the curse. The boy who lived likely owed his survival to the person who tried to kill him in the first place.
OK so, when the curse rebounded on Voldemort, the horcrux was Harry because Harry was the closest and only living thing,so technically Harry is the horcrux Voldemort never meant to make. I hope that answered your question.
since its me, i have a theory. lily potter was never killed by Voldemort directly. you see, James found out that Voldemort was there to kill harry, and he purposefully went to fight him. . .without a wand. he did this so that he could buy lily time to get away with harry, so he must have known he would die. . . which means he died for lily and harry. this means that when Voldemort tried to kill lily to get to harry, it was her the curse rebounded off of, hitting Voldemort. this exploded the house, which is what really killed lily, something harry would never have seen. and Harry's scar is there from Voldemort soul entering it, since he would have been the only living thing in the room. remember, Dumbledore said that only powerful dark magic could have caused him to get that scar, but he never says that it was definitely avada kedavra that caused it.
See, Harry became a horcrux since on the night of his parents death, Voldemort tried to kill him with an avada kadavra curse. The spell rebounded and took Voldemorts body.(well not really, just Voldemoert became powerless). Part of Voldemorts body/soul that Voldemoert lost that night went into Harry through the curse. This is sort of hard to explain, but imagine that the blade of magic that came from the wand went into the scar that it made on Harry's forehead. Then, the curse rebounded, and Voldemort lost part of himself, and that, just like the wand magic, went into Harry's scar, and then became part of Harry. I'm hoping this helped.