I agree, knowing you tend to go off on tangents, that you need a detailed outline or other master plan and that you stick to it closely. If you were writing it as text, it should be at least five or ten single spaced pages; if it's less, it's not detailed enough.
You want to break down each scene: what happens, why does it happen, where and when does it happen, who's there to make it happen. Bonus points for how it stems from the previous scene and leads to the next scene. For serious, answer these questions for every scene as you plan and you should be able to write something you can stick to.
How much chitchat is too much? Everything needs to either illuminate character or advance the plot. Anything that does neither, no matter how well written it is, doesn't belong there. Some writers find they lose their spark if they self-censor, so their first drafts are crazy long and they need to edit half of it out on the next draft. Others find that sticking to the plan and making sure they are always either showing further character development or propelling the plot keeps them where they need to be, so their edits are far less arduous.
Only by writing will you know what works best for you.