How was Voldemort able to create Nagini Horcrux in his current condition?

Question

It is known that creating a Horcrux is very dark and complicated magic. As per Pottermore

Horcrux, one of the – if not the – most difficult and extreme pieces of magic the wizarding world has to offer, for various reasons.

Rowling also stated in an interview (I don't have a quote though), that she believes many wizards tried creating Horcruxes but didn't succeed.

From the timing, we know that he made Nagini a Horcrux in the summer between Prisoner of Azkaban and the Goblet of Fire. Wormtail ran at the end of the Prisoner of Azkaban and then he obviously needed at least some time to find Voldie. Creation of temporary ugly-child body would also take some time, I assume.

That leaves Voldemort just a few months with some kind of his body back.

How did he manage to create a Horcrux while being so weak?

Answer 1

He had more strength once he got a body.

The Dark Lord wasn’t at full strength when he made Nagini into a Horcrux, but he was stronger since he’d gotten a rudimentary body. J.K. Rowling stated that the Dark Lord made Nagini into a Horcrux with the death of Bertha Jorkins, and she also specified that he could use a wand in his rudimentary body, so presumably he made Nagini a Horcrux while in that body for that to be relevant enough information to mention then.

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.
_{^{- J.K. Rowling and the Live Chat, Bloomsbury.com (July 30, 2007)}}

When the Dark Lord was in his rudimentary body, he wasn’t at his full strength, but he was much stronger than he was as pure spirit, and he was returned to a nearly human form.

“However, he was the able-bodied servant I needed, and, poor wizard though he is, Wormtail was able to follow the instructions I gave him, which would return me to a rudimentary, weak body of my own, a body I would be able to inhabit while awaiting the essential ingredients for true rebirth … a spell or two of my own invention … a little help from my dear Nagini’ – Voldemort’s red eyes fell upon the continually circling snake – ‘a potion concocted from unicorn blood, and the snake venom Nagini provided … I was soon returned to an almost human form, and strong enough to travel.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)}}

Once he got his rudimentary body, he was strong enough to use magic, including fairly advanced spells - he successfully used Avada Kedavra on Frank Bryce while in that form.

“He was screaming so loudly that he never heard the words the thing in the chair spoke, as it raised a wand. There was a flash of green light, a rushing sound, and Frank Bryce crumpled. He was dead before he hit the floor.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 1 (The Riddle House)}}

Avada Kedavra requires powerful magic to cast, and the Dark Lord was able to cast it while in his rudimentary body.

“Avada Kedavra’s a curse that needs a powerful bit of magic behind it – you could all get your wands out now and point them at me and say the words, and I doubt I’d get so much as a nose-bleed.”
_{^{- Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)}}

He was able to use Avada Kedavra, a fairly advanced bit of Dark magic, in his rudimentary body despite not being at full strength. Similarly, he was also capable of doing the magic required to create a Horcrux, though he wasn’t at his full strength.

Answer 2

Rowlings never goes into detail about what magic exactly is involved in creating a Horcrux. But we do know something (from this interview) about the murder of Bertha Jorkins, who Voldemort killed to make Nagini a Horcrux:

Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued

Thus, we know that he did have a rudimentary body at the time. He also had plenty of previous practice with creating Horcruxes which doubtlessly helped. Having Wormtail as an assistant probably helped too.