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There are plotters, and discovery writers. You sound like a plotter. There's nothing wrong with that. Take the time you need to outline your story so you feel comfortable with it, and additionally ...
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#2: Initial revision
There are plotters, and discovery writers. You sound like a plotter. There's nothing wrong with that. Take the time you need to outline your story so you feel comfortable with it, and additionally accept that things will change as you go. There are many different methods to creating a plot, and none of them are wrong; you just have to figure out what works for you. The [Snowflake Method](http://www.advancedfictionwriting.com/articles/snowflake-method/) and the [Hero's Journey](http://rads.stackoverflow.com/amzn/click/193290736X) are two structures/processes which I've recommended before. What you _should_ be wary of, and what happens more often to SF/F fantasy writers, is Worldbuilder's Syndrome, where you spend so much time creating the world for your story to be set in and polishing all the details that you never get around to writing your story.