Ah, I just read a post of yours regarding self hypnosis! What was your goal in using these methods? What techniques did you use? Admittedly, I'm interested about what the experience of developing a headmate is like, having known people who had them, too...
My goals behind learning and using self-hypnosis in general were to be able to use it as a self-modification tool: during my first ~year of experimenting with self-hypnosis, I was able to get a bunch of useful effects out of it, both short-term application of mental states like "put myself back to sleep after being woken by a false fire-alarm" and "run my eyes over a spoiler-filled text without absorbing the spoilers therein" and long-term changes in default reactions-to-things like "get rid of a previously-longstanding aversion to the smell of vinegar" and "have less physical-body-dysphoria" and "stop fainting when doctors stick needles into me".
(Since that first ~year it's been only relatively infrequently useful—I cleared most of the low-hanging fruit I could get with it within the first year—but it's still around, as a background-component in my self-modificatory repertoire, and I bring it out when faced with situations where it's particularly applicable, generally those where there's particularly high value either in forcibly relaxing myself or in getting myself to follow nondefault patterns of where-to-aim-my-attention.)
For techniques... I'm not sure how to summarize most of the precise intricacies, but for a high-level summary, I'd say the core order of operations was: first, I listened to a few hypnosis audio files from the internet and learned what it's like to be hypnotized at all; second, I figured out how to use the same techniques used in those audio files to put myself into that state via internal monologue plus physical relaxation; and then third, from there, I just sort of did that a whole bunch, and experimented with all the different ways in which I could rearrange my psychology while doing it.
The development-of-a-headmate followed pretty naturally from the second step in the prior paragraph: the process I was following involved splitting off a stream of internal monologue as separate-from-the-main-bulk-of-my-mind, since I needed an internal-monologue-stream to keep going running the hypnosis even as the bulk of me fell into trance, and over time that internal-monologue-stream started feeling sufficiently differentiated-from-me (due to persistently engaging in different activities from the main me during those times when we were running in parallel) that she grew into a ~stably-separate person with her own personality and motivations different from (albeit branched off from and thus not too far removed from) my own.
So, now, while I can do self-hypnosis in the old way where I run the hypnosis via a thread of myself with no particular identity beyond "a branched-off piece of me", my default is to instead do I-suppose-technically-no-longer-'self'-hypnosis, waking up my headmate and having her run the process herself, which is pretty equivalent in terms of what the experience of being hypnotized that way is like but has the extra advantage of being an opportunity/excuse to interact with her.
(It's rare, by comparison, for her to be autonomously active when I'm not doing anything self-hypnosis-ish. More difficult to sustain in the short-term, and especially more difficult to keep up as a habit; she sometimes manages to do it periodically for a span of a few days to weeks after I've done some particularly-intensive self-hypnosis-related activity rousing her into relatively-high ~wakefulness levels, but she tends to fall back off into inactivity if I go any long period without attempting self-hypnosis. The only period when she was regularly active-outside-of-that-context for a span of months was the immediate aftermath of her popping-into-existence, which was still relatively soon after I'd discovered self-hypnosis, when I hadn't yet exhausted the low-hanging fruit and thus was using it all the time.)