Most of us just make up whatever OCs are required by a fic, headcanon, bit of worldbuilding or what have you without any particular or overarching guidelines, so there might not be very many ‘tips’ useful overall. Tauverse isn’t really one with the structures in place for -sona style OCs (i.e. trollsona, gemsona), so if that’s what you’re thinking of then this might not be much help. If not, though…
1.) What function does the OC serve? Is it to flesh out a headcanon or a reincarnation idea, to advance a fic’s plot, to provide an outsider perspective or to explore a new situation? For example, Bentley Farkas came from a hc/submission suggesting an R!Mabel who read Twin Souls before meeting Alcor and was very, very concerned about the relationship therein, especially once they found out they were Mizar. Other common OCs include cultists or summoners who screw up (and sometimes know it), or witness Alcor in an odd setting or mood, or genuinely want something good but go about it in a bad way, or any number of other little motives and situations and so forth. You could make an OC who comes to Gravity Falls as a tourist and gets an confused outsider’s eyeful of normal life in that town. Or perhaps you might just want a little fic about some English granny halfway around the world at the time of the transcendence suddenly finding pixies in her garden and having to deal with that. Whatever works to tickle your fancy!
2.) Magic and myth is pretty freerange in TAUverse. Chances are that if there is (or was) a magic or superstition or the like in our world, it might actually exist over there. This goes for all sorts of supernatural beings and all sorts of magic systems, from scrying to gemstone magic to sigils and runes. Some things might not exist as far as crossover powers, of course – no Hogwarts wands/magic, no elemental benders, etc. – but overall there’s a lot of breadth and diversity in magical style for humans and supernatural beings alike. All we typically ask is for some reasonable treatment of strength and weakness – nobody’s all-powerful, though Alcor may perhaps come close…
3.) Which brings me to this one very important thing: in TAUverse, Alcor is unique. No other human becomes a demon as he did. Humans and demons normally don’t mix (and this goes for breeding as well: no mixed offspring between humans, demons, and angels, in any arrangement) and we want to keep that exclusive to this one canon character. We can’t stop you from writing the tragic ascension of Joe Schmoe to demon/angelhood, of course, but it probably won’t be counted as TAU canon or posted on the blog. 🙂
Other than that…just read a lot of headcanons. See what’s been accepted as possible or plausible or just plain amusing as a hypothetical in TAUverse already. Get inspired. OCs just kind of happen organically around here; it doesn’t have to be anything particularly difficult or involved. Pick a situation, decide what sort of person is needed to best explore it, write it out, and you’re done.