how does one form a stel?
oooo, that's a tricky one. I think it varies a lot from constari to constari. I'll describe my own personal experience, but I appreciate if anyone else could chime in!
For me, stels form when I latch on to a certain concept/species/creature/etc., mostly as an ADHD thing. I'll just sort of starting thinking about it a lot, want to create art about it, and eventually start associating it with myself.
Alternatively, I sometimes skip straight to experiencing nonhuman characteristic one day, and have to trace it back.
As to intentionally developing a stel- I'd imagine you do the same steps without the involuntary attraction to the idea. I wouldn't say I 100% choose to start a stel, but I definitely guide stels when they form (normally by going "mm nope don't like this aspect cut it out" and normally that gets my brain to back off).