I've tried a couple methods, but I've settled in on this:
First I create a basic concept. Just one or two paragraphs with the central plot, and the general style and genre I'm going for. Then I begin fleshing out who my central characters are going to be (main protagonist, antagonist, and major supporting players). Once I know who my players are, I begin fleshing out my basic plot synopsis into something a bit more detailed. I also begin developing the world itself. Geography, history, etc. I try to get as much of this developed in advance as I can, allowing for things to change unexpectedly (characters going off the rails on because their development takes them that way, introducing new characters or settings I didn't anticipate, needing to make an addition to fill in a plot hole, etc.). Depending on the work I MAY not have the entire world fleshed out, just enough to work from, all handily noted down so I can expand on it where needed and without contradicting myself in the book. Additionally, if I need a map I begin that process here.
Once I've got this started, I make a general outline noting each chapter and what will happen in it, IE who is involved, what plot points I'm hitting, etc. I may also make notes on specific moments I want to use (for example, on my rewrites for the novella I'm working on I outlined an entire card game so I can track who has what cards, who wins each hand, etc. so I don't get lost while actually writing it the scene). Depending on the complexity of the story and how many simultaneous plot threads I have, I'll also create a timeline so I can note where each event takes place, how long characters are taking to get from Point A to Point B, etc. Also handy for tracking things like lunar phases, tides, weather, the seasons, (if the book is set over an extended period of time) and other things that can affect the environment.. I may or may not keep the outline and timeline in separate documents. It depends on how complicated the story is; if I need to closely track them simultaneously I keep them separate.
After completing the outline, I begin the first draft.