For me it is very simple, when I am driving, in the shower, taking a walk, or any other task that helps me think, I just daydream abit. Once the daydream gets into something I can use in a story I write up an outline.
Now, the ideas that start the daydreams can be really strange, like I wrote an outline for a dark fantasy story (of which the rough draft has not been finished and is still over 300 pages long...) by thinking, wouldn't it be cool to have the sort of protagonist and antagonist fighting a war story from Mobile Suit Gundam Seed set in a fantasy environment. Then I thought, unlike Gundam Seed, lets make the antagonist really evil, but still just as relatable too as Athran was. Then I thought, lets make it a love story, then I thought, lets push the philosophical question of 'is evil borne out of love really evil at all?'; Then I thought, lets also not let them win the war, even though they are on different sides, let them join up and then lose the war. Then since I was reading some really dark stuff at the time, I thought why not make things really gritty and bleak leading to some matrydom that doesn't actually accomplish the objective. The end story, has almost zero resemblance to Gundam Seed at all, except a general level that a war is happening and it focuses on both sides.
I use an olympus voice recorder to capture my daydream ideas into something that isn't so easy to forget, as I have hundreds of ideas that I seem to simply forget if I don't follow up on them.
Now, the outline itself is written by just taking the characters I was thinking of in my daydream and the scenarios that I had from my daydreaming, and writing them down. Once I have them down what I do to finish the outline is what I call "connecting the dots". Basically I define a well fleshed out history and personality for almost every character even though I don't use most of that stuff in the story. I also make the maps and the details of the world in this stage. Once I know the character though, and I know some situations I want them in, basically the characters themselves tell me what they would do, and how they get from a to b to c, and so on.
Once I have a pretty good outline, that has all of the situations and characters I want, I will write the details into a rough draft, again this basically writes itself at this point because the story is already there. In fact, the method I use basically presents only one real challenge to me and that is that I end up making so many good characters and so much world information and such that I feel obligated to start writing those characters stories as well. I have to make sure not to write these neat characters too much into the main story lest I became so sprawling in my story telling that I start to resemble game of thrones lol