Peks' Tale grew out of one scene, really. I have a whole list of ideas that I wanted to use for "knights." However, only one of them actually ended up used. But that led naturally to how I was going to build to it. I didn't outline the later part until after almost all the first part was written. It was little things that came up during the writing of the first part that gave me the ideas for the end.
The main thing for an outline is (and yeah, it sounds obvious) just knowing where you're going. What are you starting with? A character? A scene? A theme? A world? I'd do my outlining slightly different based on each of those.
With a character, I'd look at the issues most important to that character. So I'd get soem ideas for scenes that would develop those issues or character traits. A character is rarely in a vacuum, and you probably also have related characters that come up as you talk about the first character, so use them in those scenes. Then you need to connect the scenes. do that by deciding what the scenes you've got point to as a conclusion, and then drive all the connections to that conclusion.
A scene also can't exist in a vacuum. It has to have characters, so you can do the above with those characters. but Since the scene was your idea, maybe you want to make that the most important. What sorts of events could lead up to that scene. What are the consequences of the scene? keep making new scenes forward and backward till you have a good starting point and something that concludes it all.
A world is going to have a major event somewhere: a war, a succession issue, a great evil, a romance. The "world" can be small, like a town or a villa (maybe the event is a murder or a strange visitor or the discovery of a secret), or it can be a literal world with many nations (war, disease, social change), or even a collection of planets or systems in science fiction. Tell the story of that event. Or just find characters that are influential and interesting enough in your world to write about.
With a theme, you can go several routes. Create a character who embodies the theme, or make up a couple scenarios or settings that would communicate the theme. THen look above.
Naturally, this isn't all the ideas possible, but it should be enough to get you started. If you need more specific help, we'll need more than "how do I outline?"
