"conscious mind", your direct conscious thoughts that you are aware of. Essentially the ego. Not to be confused with conscious decisions or actions, merely the thoughts.
"unconscious mind", thoughts that are not immediately or directly aware to your conscious mind, or primal thoughts that are made aware to your conscious mind that you can then choose to acknowledge or ignore. Essentially both the id and super-ego.
Tulpas are largely considered a living entity, separate and independent from their host. Rather, I believe a tulpa is a skill, like learning a second language. You learn the language of your mind and utilize that to give life to your unconscious thoughts. So many people in the tulpa community are bewildered and confused by tulpas, and usually dislike dehumanizing them for the sake of better understanding them. But I'm going to write this theory as a new way of reasoning about tulpas. I'm not implying people should consider their tulpas as non-sentient entities, as that defeats a major point of them. But what I am writing this for is so people can have a different concept of how tulpas work "under the hood" and broaden the horizon of how people think of them scientifically, or at least critically. This is simply my own theories and how I understand tulpas.
When someone decides to create a tulpa, they decide, consciously or not, that they will allow their unconscious mind to make it's own decisions, unrestricted by your self-critical conscious mind. The host creates the concept of this "tulpa", and holds no restrictions on it. This is radically different from anything else the mind has ever done. Normally, when the mind creates a concept, it has a specific goal in mind. It may be a character, or a story, or whathaveyou. The unconscious mind has no direct say in how these concepts change, because you have intended goals for them that you consciously uphold. When you create a superhero, your unconscious mind may give you an array of suggestions, like "give him a cape", or "what if he sat around watching television all day". You want to create a superhero, so your conscious mind pays attention to the first suggestion of a cape and ignores the second suggestion, as it normally has no relationship to your expectations of stereotypical superheroes. The unconscious mind, in this sense, is a primordial soup of random, incoherent ideas, that you only take a few from to bring to your conscious mind.
For people creating a tulpa, who start out with a specific form and/or personality in mind, they may find it deviates from what they intended. If it does, that means they allowed their unconscious mind to take control of this concept. They did not ignore it's first "suggestions" and preferred a different one (like the ones they started out with), but rather simply decided they would not uphold their conscious ideas of this concept. The concept of their tulpa seems to mutate by magic, because these unconscious, hidden thoughts are rising to the surface of your mind, and you're learning to listen to them through the medium of your tulpa.
And, for people who don't experience deviation, well, guess you just really know what you want.
However, the unconscious mind is not coherent enough to invent an entire tulpa. It may be able to pick out your "favorite" things, things you genuinely like, and not things you only consciously "think" you like, but it's not coherent enough to pick out every response or reaction your tulpa is expected to have. Because you have no expectations.
The conscious mind works on expectations, as I explained above with the superhero example. It both consciously and unconsciously accepts and ignores suggestions from the unconscious mind, using expectations. What you expect is what you accept from the unconscious mind, and what you don't expect, what contradicts your expectations, is what's thrown out.
Your very personality operates on what you expect yourself to act like. You may have performed an action, or had a thought, and thought to yourself "that isn't me, I wouldn't do that". Your personality takes concepts from your unconscious mind, only ones that match up with your expectations, and accepts them into the conscious mind.
The second step to creating a tulpa after the form is the personality. So essentially what you must do is learn to create new expectations, not for yourself, but for a second personality, similar to a fictional character. But you aren't consciously creating the expectations, like the superhero example. Instead, you're allowing your unconscious mind to set these expectations, as if your unconscious mind was picking itself up from it's bootstraps, creating a coherent conscious mind from the pool of unorganized thoughts that is the unconscious mind.
But what kind of personality does the unconscious mind create? For some reason, this seems to happen differently from the unconscious mind creating a form. It's more difficult, more advanced. Some people just wait for this to happen, usually without seeing results for a very long time, if ever. Other people may simply work with their unconscious mind to set these expectations they need for the tulpa's personality.
Some people degradingly call this act "parroting", echoing a phrase from ancient tulpa-making guides. The same guides that have such wonderful, insightful advice like "don't sleep while thinking about your tulpa or they'll be sucked into a nightmare world" and other such nonsense that are non-applicable to everyone who isn't the person who wrote the guide.
This "parroting", which I prefer to call "daydreaming", is the act of consciously setting expectations for the tulpa's personality, by allowing the unconscious mind to control these expectations, since you're actively not setting any standards for them, like the superhero example. Your unconscious mind cannot think, it cannot daydream. it can only really do these things when you're unconscious, in other words, sleeping and dreaming.
Therefor, some people "daydream" their tulpa. They could directly daydream about their tulpa acting and moving, or they could do it discretely by simply thinking of (or "wondering" by asking their unconscious mind and simply taking the first idea that comes to them) the first thing the tulpa would say to a response.
This is what I call the unconscious mind picking itself up by it's bootstraps. You could think of it as holding the baby tulpa as it learns to take it's first steps. It can't quite stand up by itself, or think for itself, but by you consciously controlling it, but taking instructions from your unconscious mind, you're supporting it as it learns to stand for itself.
This is, what I believe, how people gain tulpas from writing fictional characters. They actively daydream about it and give it life in their heads, and consider "what if they were real?" But instead of the poisonous "No! I can't 'parrot' them, that's witchcraft and immoral!" they happily daydream about them and "consider" what their responses to confrontations, building up this character's personality and expectations from their unconscious mind.
But, everyone is different. It doesn't matter so much how you build up the tulpa's personality and expectations. All that matters is it's "in sync" with your unconscious mind, it's made out of thoughts generated by it, and thus the expectations match up with some of the random thoughts it generates. As long as at least some of those random thoughts are "the tulpas" just as some of those random unconscious thoughts are "yours", meaning they would be thoughts you would choose to consciously think or do, then the unconscious mind will be able to supply the tulpa's expectations with thoughts to generate it's personality. A fictional character you didn't create can't be your tulpa, because it didn't come from your unconscious thoughts, none of those unconscious thoughts match that character's personality. Unless you really like that character and project onto them, then it's free reign.
As the personality is forming, the tulpa may begin to appear "sentient" and "independent", it may start having ideas or thoughts separate from yours. In reality, you and your tulpa are both taking thoughts from your unconscious mind, but different thoughts from each other. A thought you didn't see or that didn't meet your personality's expectations might have been picked up by your tulpa, and it may indeed surprise you and shock you that it knew this thing or could think of something you didn't. You did in fact "think" of that thing, but not consciously, and you unconsciously refused it, but your unconscious mind decided that thought would work for the tulpa's personality, and so it brought it to your attention anyway.
I can't quite say how exactly the mind "runs" this personality alongside yours. It may be different for everyone. On average, it indeed seems to be a conscious act, as you can consciously ignore your tulpa and pretend it doesn't exist, and it stops existing. But when you consciously allow this personality to exist alongside you, it seems this enables your conscious mind to see these unconscious thoughts that don't match up with your personality, but rather your tulpa's. This then enables you to consciously or unconsciously imagine your tulpa performing or thinking the action or thought that came from your unconscious mind that matches it's expectations.
Sometimes, especially during the tulpa's development, you and your tulpa may have the exact same thoughts. This may feel as if you're unintentionally controlling them ("actual" "parroting" if you will). This is not so. It's simply that the tulpa's personality and expectations are young and incomplete, and is relying on your own expectations to do some thinking, thus both you and the tulpa accept the exact same thoughts from your unconscious mind.
This is all simply a skill. All you're doing is learning how to unconsciously control a concept, your tulpa, separately from yourself. Like patting your head and rubbing your belly at the same time. You learn to listen to your unconscious mind and feed your tulpa with the thoughts that went unused by your conscious mind, yourself. Normally, people get minor headaches from forming a tulpa. Headaches are a common symptom to learning a skill that requires any new method of thinking and comprehending something, as the brain is literally rewiring itself and forming new connections to different parts that perform different functions. The same thing happens when learning a new language.
This ability to tap into the unused parts of your mind, your unconscious thoughts, the ideas you unconsciously or semi-consciously discard in favor for thoughts that are more acceptable to your critical conscious mind, is one of the more useful and beneficial uses of a tulpa. Normally, when people want to tap into their unconscious mind, they'll either take drugs or dream, which removes the critical expectations of your personality and allows unconscious thoughts to flow freely. But this method allows one to consciously listen to their unconscious mind while sober and aware. Not directly, but through proxy of the tulpa, which is, in reality, your own skill in listening to your unconscious mind through proxy of a personality separate from your own, as a net woven from the strands of your unconscious thoughts. I've witnessed this first-hand as my tulpa has given me ideas that I genuinely did not think of or even consider, which surprised me.
In essence, a tulpa works, thinks, by using the thoughts you didn't think of.
As I've stated in the first paragraph, I don't mean to imply that people should be considering any of this when actually interacting with their tulpa. A tulpa, to you, is meant to be a living thing, with thoughts and emotions of it's own. It would be bad to break that illusion, as it could inhibit it's functionality. Rather, I think these theories, if accepted by the larger community, should be taken into account in developing new tulpa-making guides or methods on creating tulpas or servitors. Yes, a tulpa may just be you recycling unused thoughts, and emotions are just a bunch of chemicals, but it's the first-hand experience that makes it human to us, not the technical details.