While each writer had a slightly different way they went about things, we all generally followed the same idea.
First off we made a scenario plan. It was a very general description of what major events occur and what form the story would take, and largely served as a way for both the writer to stay tethered to a plan without wandering off into the wilderness on a whim (which happened anyway, in most cases) and as a way for others in the development team to critique the path's foundation as presented before too much work was wasted.
Then we wrote out the paths. We wrote generally scene by scene, and in chronological order, with each posted to the forum as they were done and staff leaving a comment or two on any scenes they wanted to comment on so things could quickly be addressed. Each scene was written in whatever program each writer used (I just used Notepad), and took the form of a simplified Renpy script (no/very little direction, comments where wanted preceeded with a # to mark them as such, each line keeping to the length constraints of the in-game textbox). This part is what took up most of the path's writing times.
As this happened, Delta (and eventually some of the writers) inserted the scripts into the
SVN repository (very basically an online copy of the game with the previous versions tracked). This meant that the dev's local versions of the game could be updated and the scenes read through in-game (though without sprites, backgrounds, music, sfx, etc) and any lines going outside the limits of the textbox could be fixed.
Then we scrapped pretty much everything and began to rewrite the paths.
The first drafts were far from useless - they served to lay down a first idea, as unrefined as they often were, of the path, and gave the writers valuable writing practice. After they were finished, the developers read each completed path and gave complete feedback and collected thoughts on what they were like (especially Aura, as head writer). Some needed just a bit of work, others needed quite a lot. Using this feedback, the writers produced a new plan for the second (and final) draft of their paths, this time a much more detailed scene-by-scene plan detailing what each scene would entail. This was also commented on by each developer and eventually laid the basis for the new second draft paths.
As the second drafts are being written (which is what is currently happening in Devland), they're being inserted into the SVN usually as they're done and then edited in there by the editors (Silentcook, Kagami and Losstarot). After each act is edited, Delta goes through and directs the scenes, adding sprites, backgrounds, SFX and music cues as well as fancy effects (for example, Emi's bouncing and the Shizune fireworks in Act 1, among others).
I guess my main advice would be to not be afraid of throwing out work. Writing can be cranked out on a good day, and sometimes a bad idea just isn't worth saving. And as always, practice, practice, practice.