I feel like getting in on this action. Sumiko's Character Creation Guide (with examples!)
You want an idea for an OC. This can be an idea for abilities, for personality, for development, or all of the above. This can be a unique idea or a re-creation of something else. I started Sumiko as an idea for a zanpakuto ability, Tamsin was based off of a character in a book I read, and Alexis is based off of my daydreaming to be assistant to the powerful.
You want to flesh out your idea. Fill in the areas you're lacking. Sumiko's zanpakuto ability needed a scientist to properly use it, and mad scientists are fun. Tamsin finds her left arm useless right now, so she's totally gonna lose it again at some point. Alexis' abilities are the result of me smashing in an abilities idea to kill 2 birds with one stone.
I find it vitally important to look over the documents for the race you wanna pick at this point. You're not hammering everything out just yet, but it's good to make sure things seem to fit together.
I like to include appearance at this point as well. You can either decide an appearance first and go for a picture later (like I did with Sumiko, though I ended up having to draw her picture myself), or you can look for pictures that you think fit the character (like I did with Tamsin and Alexis; it's easier to have multiple pictures this way)
By combat stuff I mean stats and abilities. Your starting idea may have been an ability or ability theme, but you wanna fill in the magical tables of Name, Type, Cost, Stat, Range, Cooldown, and Description. You may have to make more than one ability, depending on race. You can also make more abilities than what you start with so you've got the theme down. Make sure they make sense together, and try and vary it up a bit as well. Having 3 offensive abilities will be tough when you're on the ropes, but if that's who your character is, then that's what you can do. Sumiko had a utility-type shikai and only had offense/defense in her bankai, I dunno what I'm doing with Tamsin, and Alexis has got a bit of everything.
Stats are fairly simple, just make sure you don't set anything to 0 and also don't break caps. The main doc will help you with that. Do something that makes sense, especially with your boosts because those can't be altered. Don't worry what numbers you put where too much, though. You'll be earning points quite quickly to start.
At this point you should be good to submit, but you can do the following stuff before anyways.
This is really awesome to do while waiting to get approved. Find some questions to answer to get all the tiny details of your character. Hammer out a backstory, think back to what events were important to your OC. Sumiko was experimented on as a child, Tamsin had a family, Alexis was a rebel. Describe their personality as much as you can. Look at the wiki template and write down what it needs so it's easier to do when you do it.
Well, not really. Submit your character, get it approved, make your wiki, request flair and approved submitter status. Then you should be good for an intro post. You can have your character just walking around, but try and have them do or say something that gives a simple idea of who they might be. After that, well, then you'll be RPing!