The drawing part is completely up to you - it can be whatever size and format, but the drawing depicted should be at least full-body, so those wishing to draw your character have a ref. If you are a writer - describe as best as you can.
The Celestial spirits folder is there to offer you reference! Check it out.
For description part - follow the character creation part of the
Rules journal and copy/paste the form linked there and fill out!
When your APP is ready and submitted to DA, press the join request button and post a link of it in description. And that's it! (If any corrections are needed, I will comment under your join request)
If you have more questions, feel free to ask. *v*//