Okay so this is basically my interpretation of how horcruxes are made (written to be like a short book or guide), except a version that even someone who hasn't read Harry Potter will get. As long as you know magic exists, you're good. The story behind that is that I actually turned this in as a process essay for my English 101 class, and my teacher was very set on having it be easily understandable. This means that things are slightly simplified and also slightly censored. I might upload a longer version sometime where I really get into the topic, but for now this is what I've got. Hope you enjoy it!
*I don't own Harry Potter or Horcruxes, just the steps and theories included here that go beyond the book*
Horcruxes are one of the darkest magics to ever exist, allowing the user to evade and even conquer death. The basis of this magic is to anchor a piece of your soul to the earth in some material object, therefore making it so that the piece of soul still residing inside of you (henceforth referred to as the master soul) cannot pass on even when the body is killed. The process of creating a Horcrux is challenging and not without its downfalls, but for the determined wizard it is extremely rewarding.
There are many reasons why you could want immortality. For some people, it gives them the ability to spend more time working towards their goals, whether they are creating a new potion or conquering the world. For others, it gives them the ability to live without a sense of self-preservation. There may be consequences to your actions but in the end, you cannot truly be stopped, leaving you free to do as you please. Some people might even just want to have the satisfaction of knowing that they cannot die, since there is no feeling more powerful than that of immortality.
The first step in this process is to prepare the vessel in which the removed piece of soul will be placed. Any object or living being will do, such as rings, snakes, or even just a rock. Once the soul fragment is latched onto it, the object takes on the indestructible property of the soul, meaning that you can use almost anything. After you have chosen something to put the fragment in, you must make sure that it cannot get back out to rejoin with the master soul. The piece of magic used to accomplish this is called the Soul's Binding spell. This spell will act as a kind of trap, by drawing the fragment into the object and causing it to lose its ability to rejoin with the master soul. The soul fragment is fundamentally different now, as it is bonded with the object it has been placed into. This means that if the object were to be destroyed somehow, the soul fragment would just dissipate.
When executing this first step, it is important that you do it in a secluded space like a forest or a dungeon. Any magic involving the soul is very powerful and unstable, and could harm anyone around you. The incantation for the Soul's Binding spell is Lateri Animae. When you say this incantation, you will wave your wand in a circular motion over the chosen object. The object should begin to glow brighter and brighter, but do not stop no matter what. After about a minute there will be a flash of golden light, signifying that the vessel is now prepared to house a soul.
Now you must move on to the actual creation of the Horcrux. It is not important when you do this in relation to the preparation of the vessel, but once the procedure has been started you must work quickly as everything happens in very rapid succession. The result here will be that you have done something to split your soul, and then captured the piece that has split off before it can rejoin with the master soul. There is only one act so foul and heinous enough to have the power to split one's soul, and that is murder. The killing must be intentional, and done through hatred. The messier it is the better. Drawing it out or using torture makes the deed even worse, leading to the soul fragment staying split longer. This allows you to capture the piece easier. Any person will do as your victim, but the fact that killing is required gives you a marvelous opportunity to deal with any enemies or dole out some revenge, thus getting two tasks done at once.
Most people cannot normally sense their soul, but something as intense as a split will most definitely be felt, both physically and emotionally. It will almost feel as if your entire being is painfully ripping apart, the very essence of yourself disintegrating. It is a pain like no other, but worth it in the end. The split piece of soul will be flung out of your body, but will almost instantly try to reintegrate. You must use this small window of opportunity to capture it and force it into the chosen vessel. The spell used here is a more powerful version of the summoning charm Accio, called Accio Maximus. You should wave your wand in a large arc, causing the soul to return to you without letting it reintegrate. It is important to note that everyone views their soul differently so it is impossible to say what you will see, but it should be recognizable since it is a piece of yourself.
Now that you have control of the soul fragment, you just need to get it into the prepared object. This is probably the easiest part, since all you have to do is direct it towards the object. The fragment should currently be hovering near your wand. There is no exact spell involved here, just your force of will. Directing your wand towards the object and envisioning the transfer of the soul from your wand to the object is all you need. The soul piece and object should bind with an audible cracking noise. Now your Horcrux is complete.
Lastly, you will need to find somewhere safe to keep your Horcrux. Most wizards prefer to keep it in a place they can access easily for peace of mind, such as in a piece of jewelry to be worn, but if there is any threat of it somehow getting destroyed you might want to take extra precautions towards hiding it. There are innumerable options here with what you can do, from hiding it somewhere in your home to putting it at the bottom of the deepest, darkest ocean.
If you are especially ambitious, you could even create more than one Horcrux. The problem here is that it has never before been attempted, so we can only speculate on what would occur. It seems possible that you could split your soul more than once, but each time you did so it would become more and more unstable, as a soul that is only a third or a fifth of what it used to be can only become a bad situation. I would advise anyone attempting to make multiples to have long intervals of time between the creations of each one, and to be extremely careful as you are doing something incredibly dangerous and never before attempted.
While the creation of a Horcrux is a very rewarding task, there are also some downfalls. One is that if you are ever killed somehow, you will not be able to pass on to the afterlife. Only complete souls may do so. The only way to reform your soul would be to feel true remorse for murdering your victim, but this is an extremely painful process and no one who has made a Horcrux would want to give up his or her accomplishment of immortality. Another effect is the aforementioned instability of the soul. It is unknown what the effects on any specific individual would be, but some possibilities are physical disfigurement, a loss of one's humanity, and even the chance of the soul being so fragile that it splits of its own accord.
Even with the risks involved, creating a Horcrux is a truly worthy challenge for those seeking power. A true dark wizard has no fear of the consequences, as they pale in comparison to the potential of living after death. The benefits of this magic reach far into the future and can continue to help you throughout your entire life.