(Not even close to being finished yet)
Akhiloth's tulpa guide

Hey, welcome to my tulpa guide. Happy you chose me. If you're reading this then you were probably either just done reading someone else's guide, or this is your first. Regardless, I hope it serves you well, and that you continue researching deeper into the tulpa phenomenon after being done with me.

Usual warnings are in place, such as that nothing that this guide says can make me responsible for any potential harm, or that everything you read is to be taken as only one perspective among many others. For everything else, just stay rational and calm and you'll do fine.
Part 1: My personal understanding of what a tulpa is

There are several functions that the mind fulfills. One of them is the separation of the self and the other. Without the mind interfering, reality is obviously one. There is no self and no other. The mind makes this distinction. A tulpa, in my own terms, is a personified manifestation of the mind's perception of the "other". We tend to impose a hierarchical relationship on tulpas by defining the ego that came first as the "host" to them. These roles are fully interchangeable, since there really are (very few) reported cases of a permanent switch between a host and their tulpa having occurred.

One must be careful not to refer to psychological phenomena as occurring "within" the brain. Strictly put, the only thing that ever occurs in the brain is the electrochemical impulses between neurons. The brain is an organ, subject to rules of physics and chemistry. The mind is a method of describing subjective experience with concepts. Due to the mind-body problem still being around, equating the two isn't possible. That is why I never talk of the "brain" or the "head" as housing the tulpa.

As mentioned prior, a host is the ego that was the first to emerge within a mind, and that would later see a tulpa join it. The host first starts piecing itself together as an about two-to-three-month old baby in the normal process of psychological development during infancy. Throughout its lifetime, the host goes through numerous changes, eventually finding itself in a situation in which it is motivated to create a tulpa. In truth, this motivation itself is that which creates the tulpa, but the ego-illusion makes it seem as if the host is directly responsible, instead of merely picking it as an option. Nonetheless, the mind's perception of the other is molded into such a shape that an another person emerges.

The normal perception of the other is not entirely disturbed upon starting the process of tulpa creation. With an intentional decision on the host's part to look out for signs of their tulpa, the host begins picking out parts of their experience of the other to construct a person out of. Even though the host begins perceiving an increasing number of phenomena as being their tulpa or the result of said tulpa's actions, only a small part of reality is so transformed. The host will not begin experiencing the entirety of their subjective reality as a tulpa, even though the potential is there.

Since the tulpa is first and foremost a being and not an archetype, the host typically chooses with their intent the personality, appearance, and general "vibe" of their tulpa to be like a person's. The key factor, however, is sentience. The tulpa could also be an animal or a rudimentary lifeform such as a sentient ball of energy. What is important is that the potential for sentience is accepted from the very beginning. The tulpa is supposed to end up sentient, so it must be accepted that they are capable of sentience from the moment that the decision to create a tulpa is made. This is not to say that they ARE already and without a doubt sentient. That may or may not be the case. What must be accepted is that one cannot know if their tulpa is NOT sentient, but they CAN know if they ARE. One may be able to speak, but chooses to remain silent. This applies to both someone with access to a physical mouth and someone without.

A tulpa grows more complex and independent as practice continues. This is analogous to learning a skill, such as playing the violin. The mind has to reorganize itself to form new habits that result in the desired tulpa. There are various learning techniques (often called "methods" in the tulpa community) that may be employed to ease the process.
Addendum about Tibetan Buddhism and how it relates to the western understanding of tulpas

If one is to compare the average tulpamancer to a vajrayana practitioner, they must consider the motivation that drives Buddhists forward and contrast that with what inspires a person to create a tulpa.

Buddhists seek nirvana, first and foremost. Depending on if you're a Theravada Buddhist or a Mahayana Buddhist, you may be more or less concerned with who it is that's attaining it. Theravadas are concerned with the personal liberation of individuals from samsara, while Mahayanans strive to become Boddhisattvas: beings that help all of reality become liberated.

What is nirvana, exactly? That question cannot be answered, as nirvana is beyond any concepts. Given that we live in samsara, which is conceptual reality, giving an example of that which is not nirvana is something that we cannot avoid doing every time we speak, because language itself relies on concepts. Nirvana exists in relation to samsara, the conceptual realm, even though the two are ultimately the same thing. You can lead others or be led yourself toward nirvana from samsara (which is the point of Buddhism). The best way I've found is to explain this relationship is with an image of a potter's wheel.

The wheel represents the possible range of experiences and emotions. The closer a piece of clay is to the edge of the wheel, the stronger the the pull, or in other words the stress, that is acting on it. This correlates to the varying strengths of different emotions. If a piece of clay was to sit at the exact centerpoint of the wheel, it would be completely motionless. Parts of the wheel that induce force are samsara, while the forceless center is nirvana. Also notice that looking from the center, the edge is always the same distance away no matter which direction you're facing.

So samsara is an infinite range of superficial understandings of reality while nirvana is the one fundamental understanding. This is because all of samsara is impermanent and nirvana is permanent. Samsara is built upon concepts, and perhaps the central tenet of Buddhism is: "All things that have put together must fall apart." Concepts are built of smaller parts and exist in relation to something else, and are therefore subject to change and impermanent.

A parable goes: "A Theravadan, a Mahayanan, and a Vajrayanan all come across poison bushes while on their own respective roads. The Theravadan avoids the bush altogether. The Mahayanan tries to uproot the bush to make sure nobody else might step into it. The Vajrayanan stops to wonder if they could make some kind of medicine out of it". Vajrayana is sometimes considered a branch of mahayana and sometimes its own thing. What's special about it is all the strange and elaborate tricks (called tantra) that are used to bring oneself and others closer to nirvana.

This is where tulpas come in.

The fundamental practice of vajrayana is deity yoga. This involves going into meditation and creating a sort of 'shell' for a deity to enter, to then invite said deity into said 'shell'. This involves visualization and the sensation of the deity's presence. After the practice is over and done with, the practitioner lets the construction fall apart and exits meditation. The reason why this is done in the first place is because Buddhist deities are actually archetypes of skillful mental qualities such as compassion, discernment, generosity, virtue, et cetera. By inviting them to visit you, you're measuring this ideal archetype against yourself and thinking of ways in which you could be more like them. You do this practice until you can no longer tell the difference between yourself and the deity, having effectively become their emanation ("avatar"). An another purpose is to watch your mind "unfold" during the meditation so that you can remove any mental obstructions easier. At no point is the meditator expected to keep this deity around to chat with.

Tulpas aren't much like this. A tulpa persists outside of meditation and can be conversed with like any human being. A tulpa isn't usually an archetypal figure and can have a vivid personality. Most importantly, a tulpa is not created for the ultimate purpose of bringing beings to nirvana. A survey conducted in 2014 by somatosphere.net concluded: "These individuals, calling themselves "tulpamancers", belong to "primarily urban, middle class, Euro-American adolescent and young adult demographics" and they "cite loneliness and social anxiety as an incentive to pick up the practice."" Quite a far cry from a Buddhist monk. The word itself is apparently an obscure term referring to mindstream emanations and isn't really used anymore near its etymological origins, and definitely not anywhere near this context.

In a book published in 1929, a western occultist by way of theosophy named Alexandra David-Néel talks about her trip to Tibet. There, she tried her best to understand the local vajrayanans' practices through the lens of her particular worldview. Her understanding resulted in her creating what might be considered the first tulpa. This jolly Friar Tuck -like fellow was rather short-lived as- frightened of her own creation- Alexandra ended up destroying him. How she accomplished this is never explained. Almost a century later, some people on the internet find her book and try replicating her efforts, much to their success. Creepypastas were written, Pinkie Pie tulpas were molested, and everyone lived happily ever after.

So the "tulpa" is now a purely western concept, stemming from one explorer's misunderstanding, although with very real, traceable connections to equally real vajrayana practices. Just not necessarily the ones we'd expect. As long as this is understood, there's no reason to feel shame over having one or wanting to create one. Even a misunderstanding can lead to an unexpected positive development.