As PS indicates, you've already got the basic plot sorted out. You probably have a rough idea of how each scene will pan out.

Don't worry about length - it could be a few thousand words, a few tens of thousands of words, or even longer. Break it into chunks or scenes, then work out roughly what you want to happen in each chunk / scene, and start writing.

If you need inspiration for the details, a story with similar themes that may help is "Only a Baby Machine". It's quite long and is a proverbial car crash of a story, but if FF (forced fem) is your game...

Another good place to start is Fictionmania - I don't frequent it, but from what I've heard from others, there's more FF over there than over here.

When you've got it (or the first segment if you want to release it in installments) written, post here again requesting an editor. It would also be useful to say what you expect from the editor - general spelling / grammar fixup or to rearrange / reword bits. Do you want 'light-touch' editing (where they just fix up the most glaring errors), something more intensive or are you prepared to accept a critic (someone who won't be afraid to say "this paragraph / concept / idea doesn't work" or "X is acting seriously out of character in this scene")?

And finally, if you do decide to start writing it, the important thing is to persevere. Although many stories have new chapters on a daily or weekly basis, some are updated monthly or even intermittently (the SPA universe is a good example) and we're patient enough to wait. After all, most authors do not sit at their word processors for 8+ hours a day, 7 days a week, creating lovely stories. They do have a life outside this community, with careers / families / health issues / household maintenance etc. that have priority over story crafting.