I work with my husband as the co writer on mine.
It started with an idea I had and with help from my husband, we refined the character and the plot a bit more.
We then wrote down some pieces of the story that we want to hit -- though it is not concrete-- as we want the characters to dictate the story to us as we write it.
Husband writes pages to an issue and I go over them with my notes. There's some back and forth for a bit and then I start doing rough layouts (where I make additional changes to the script if needed).
Often times, after he sees the layouts, he gives me more notes (and we go back and forth).
Then as I am drawing round 2 layouts, more dialogue changes can occur and we seem to shape it into something better than we first started.
(I should note, the entire script for the issue is often times, not finished when I start drawing roughs!)