Okay so its pretty obvious how to rip the Soul apart. Its not just killing people in a duel its planned and processed murder.
So how do you get the bit of soul floating around inside you, into something else?
Here's 3 possible theories.
Okay so we can yank out our memories and put them in jars and vials and replay them over and over in the Pensieve, so why can't a Powerful Evil Wizard Overlord like Voldemort just wave his wand and do the same thing?
2: If you revisit your battered copies of the Tales of Beedle the Bard, you'll find a version of the Story called "The Warlocks Hairy Heart". The guy cursed out his own hert with some seriously dark magic, and locked it inside of a glass casket. Sounds like a Horcrux right? Okay so the guy ends up cutting the heart out of the girl he ended up being in love with despite his twisted efforts and tried to steal her heart - litterally. But without his mortality in his body, be cant die.
So, in my opinion using extremely dark magic, working very much the same way as the Resurrection of The Dark Lord in GOF, the soul would leave its body, and enter the Host, or the Object basically just a really advanced possession.
Okay so my last theory rides on Greek Mythology:
Kronos killed his Dad the sky with a scythe right? Chopped the dude up into tiny bits and scattered him throughout the universe.
Horcrux step 1: Murder.
Then the guy Eats his children. Like as soon as they fly out his wife Rhea's Vagina BAM theyre breakfast. Well, except for zeus because Rhea fed him a Stone instead.
Well decades later the guy gets food poisoning and regurgitates the rock and his 5 kids who are full sized adults. Well a peice of that guys soul sticks to the Stone, giving it that weird Kryptonite power on the Gods.
So by ingestion, or metaphysical manifestation, or by waving a peice of wood around you can create your very own ticket to hell!