*makes a funnel out of hands and shouts in a shrill voice* Ceeeeeeeee!
OMG, I'm so glad you'll be writing.
Where to start? I guess with some headcanon that you really wish you could share and make it more "real" as you write it in a story. It doesn't have to be anything huge or complex (unless you want it to). I started with these 300-word long drabbles that were just banter.
It can be scary to post online at first but totally worth it. You can always have someone look it over and give advice and reassurance before you're ready to post. And definitely come and holler on the smut thread if you need support. We can be *very* enabling 
I'm thinking if I do write something, it would probably be just a short story of a particular headcanon like you said. Nothing multi-chapter or anything (because my god, I'll cry if I did that). Smut? I dunno. I have been toying around with a smut scenario I've had in my head for the longest time, though.
I think one of my problems would probably be making sure the characters actually sound like them (I'm not sure if I could write Sera to sound like Sera for example
).
And yes, I'll probably need someone to look over my stuff (and maybe give some advice or two. Or three. Or four... xD)
Start with what you want to see in a story. I didn't think I would actually post mine publicly either. I started in May writing, and didn't post anything until October. I thought for sure if I wrote it for me that would be enough, then my son wanted to read it so I put it on Fan Fic and it has grown since then.
You write for you, first of all. If you think about writing for everyone else the pressure can get overwhelming.
I think Cee can approve you for the group.
Thank you! I'll keep that in mind. 