This answer lists all the people Voldemort killed to create his Horcruxes, but I couldn't find anything on how he killed them. According to this answer, the spell to create one is separate from the murder itself, so they could all have been slain in unique ways.
How did Voldemort murder each of the victims used to create a Horcrux?
The books don’t explicitly mention who was killed to make each of the Horcruxes. Some of them can be at least narrowed down in possibilities, but others are much harder to deduce who was killed to make them from the content in the books. The closest thing we have to a list of who was killed to make each Horcrux is this list, that JKR gives in an interview.
Lady Bella: Whose murders did Voldemort use to create each of the Horcruxes?
J.K. Rowling: The diary – Moaning Myrtle. The cup – Hepzibah Smith, the previous owner. The locket – a Muggle tramp. Nagini – Bertha Jorkins (Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued). The diadem – an Albanian peasant. The ring – Tom Riddle Sr.
- Interview with the Leaky Cauldron
Moaning Myrtle - killed by the basilisk controlled by Tom Riddle.
Hepzibah Smith - poisoned, which Tom blamed on her house-elf Hokey by implanting a false memory in Hokey.
“Hepzibah Smith died two days after that little scene,’ said Dumbledore, resuming his seat and indicating that Harry should do the same. ‘Hokey the house-elf was convicted by the Ministry of poisoning her mistress’s evening cocoa by accident.’
‘No way!’ said Harry angrily.
‘I see we are of one mind,’ said Dumbledore. ‘Certainly, there are many similarities between this death and that of the Riddles. In both cases, somebody else took the blame, someone who had a clear memory of having caused the death –’
‘Hokey confessed?’
‘She remembered putting something in her mistress’s cocoa that turned out not to be sugar, but a lethal and little-known poison,’ said Dumbledore. >‘It was concluded that she had not meant to do it, but being old and confused –’
‘Voldemort modified her memory, just like he did with Morfin!”
- Harry Potter and the Half-Blood Prince, Chapter 20 (Lord Voldemort’s Request)
Muggle tramp - unknown cause of death.
Bertha Jorkins - likely killed using Avada Kedavra, especially since the Dark Lord’s ability to hold a wand is mentioned.
The Albanian peasant - unknown cause of death.
Tom Riddle Sr. - killed using Avada Kedavra, which Tom framed Morfin Gaunt for, by implanting a false memory and using Morfin’s wand to commit the murders.
“The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how the Riddles died, for the Avada Kedavra Curse does not usually leave any sign of damage … the exception sits before me,’ Dumbledore added, with a nod to Harry’s scar.
‘The Ministry, on the other hand, knew at once that this was a wizard’s murder. They also knew that a convicted Muggle-hater lived across the valley from the Riddle house, a Muggle-hater who had already been imprisoned once for attacking one of the murdered people.”
- Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)
So, one was using a basilisk, one was a poisoning, one was certainly Avada Kedavra, one was likely Avada Kedavra, and two were killed using unknown methods.
