In my earlier days as a writer, I used to just sit down and start writing once I had an opening, a bit of an idea, or felt it was time start writing a new story. This led to a lot of crappy fiction with a snippet or two of something usable thrown into the mix. Over time I've come up with a method that leads to much stronger stories while still preserving the fun part of writing, which is the discovery process.
I'll start with an idea, which may be nothing more than "time to write a new book" or it might be well developed, such an idea with established characters and the ideas of a plot that is the next book in a series. I'll open up a new document and start brainstorming. All sorts of stuff will go into this file: fragments of dialog, my goals for the book, character sketches, questions that are unresolved. Over the course of a few weeks, this document will grow into 30-40 pages. Ninety percent of this will be dead ends or otherwise unusable, but over time, an actual plot will start to come through. I'll have major turning points, mysteries, revelations, set pieces to work toward, some scenes. Before I'm ready, I like to have an understanding of the major characters, the opening few chapters plotted out, at least one major set piece where things will blow up in the middle, and a rough idea of the ending.
At that point, it's a question of setting a date to begin the actual writing. I'll go back to the brainstorming document as I work and I detect major plot holes. And I don't feel obliged to stick to the quasi-outline, but use it more as a rough guide. Sometimes (usually?), the plot takes major detours. I do find, however, that having this document to refer to keeps me from wandering down blind alleys that can waste weeks of time.