Writing Outlining: How Do?

It will vary by needs.

I have over 900K words written, and I never wrote a proper outline.

However:

While I only started with a basic scene and some characters, and just simply started writing from there, I discovered the general shape of my primary antagonist in the process. Great, I have a mental pin: At some point, a person related to this group is going to make themselves a problem. The end of the serial is when they have dealt with that problem.

As time went on and I built more world, I had more ideas to add as way points. I then discarded about half of them along the way, as they no longer fit my current development of the story.

I was also wrong about 'solving' that problem being the end of the serial; turns out, that was the end of the first major arc. The way certain other things developed, there is an entire second major arc to be spun off from there.

That should be the end of this first serial, but I also have a bunch of ideas for other series set in the same world. :)

This is one of those things where the answer depends on what works for you. I'm new to posting on scribblehub, but I'm not new to writing. I do have an established "what works for me," but it's heavily tailored towards a specific style: I publish allegorical fiction first and foremost. I'm on the extreme of the "architect" scale (rather than a gardener); I plot out my entire stories, down to the level of individual chapters, before I start writing. My outlines are usually longer than my stories, in that I include notes about what I am trying to achieve with each line that I write.

How I go about this (for anyone who wants to replicate the process) is as follows:

1. Start with what I want to say. By this, I actually mean the message. As an example, let's say I decided I wanted to write a story about how the commodification of human relationships is harmful to society. That would be the message I started with.
2. Decide on what shape I want the allegory to take. In this case, lets say I decided to structure it as a LitRPG. The system would need to be a bit "non traditional" (perhaps award "level ups" or "stats" for forming new relationships, etc). But the point would be to use the structure of the system and the societies that would develop as a result of it to serve as the shape of whatever it is I am trying to give commentary on.
3. Consider how I want the story to end. Is it a tragedy? Does society completely erode the idea of a genuine relationship in favour of a relationship that awards maximum stat points? Is it a call to arms? Does the protagonist side with or against the system? It's important to note that "how it ends" can be entirely divorced from "the message." A tragedy where the system wins is not necessarily an endorsement of the system, if it is properly portrayed as a "bad thing." Let's say, in this case, it's a tragedy where someone who is pressed down by the system and hates it eventually comes to endorse it once they get a taste of what it's like to be near the top. The story would follow their gradual shift of opinions, and show how these kinds of systems influence people over time, as well as what was lost in the process.
4. Consider what society would look like as a result of point 1 and 2. What cultures would develop? In this case, maybe people would be incentivized towards "faking friendships" because the entire system would be structured that way, so culturally, everything from education to dating to the work environment should reflect that. A lot of worldbuilding occurs here, too much to fit into a single post, but think of this as "establishing what the setting is."
5. Decide on what perspectives within this story would serve best as an engine for the allegory. This could be anyone, from street sweepers to politicians, so long as it gives you a diverse array of opinions. You need a cast of characters with varying opinions on the subject, because the point is to never lecture the reader, but to make them think about it, and that includes making them think about it from the perspectives of the people who are in favour of whatever system is being examined. Understanding why someone might be a proponent for something is important, even if your argument is against the thing they are supporting. I usually like a character cast of 6-8 primary characters each with a different opinion, because that usually gives you enough nuance on complicated subject material to make people think about it, without having the opinions overlap enough to become "samey." I'd generally have at least one complete rebel, and one complete "priest of the system," within the story, at the bare minimum, with other characters falling somewhere along the middle.
6. Start to fill in the background details for each of these characters; write say a single chapter for each of them to "get a feel for them." It's important that none of these characters are one-note. How did they come to have whatever opinion it is they have? What happened in their life? What are their goals? Give them unique speaking tics and quirks; make them feel like people. Readers will not tolerate reading allegory if you do not do this work from the get-go, because it will feel like being lectured if you don't do it. Then, once you have done that work, start to consider "how do all these characters meet." These characters, by design, will all have very different opinions, so there is plenty of room for conflict.
7. Plan out broad arcs from, "how they meet" to "how the story concludes." Think of these as narrative arcs within the larger arcs. These arcs should include conflict and conclusions. In our example, let's say the protagonist is a woman who is trying to pay off her debts by working at an uptown restaurant, who is about to be fired from her job due to not maintaining enough relationships to maintain the minimum level requirement to work at the restaurant. The story could possibly start by her meeting someone who is offering "shady" ways to make ends meet on her way home (somebody associated with the "rebel."). Arc 1 would establish all (or at least most of) the characters, and possibly end with the protagonist choosing to associate with one of the factions. Arc 2 could then test their certainty in that initial choice and instil doubt. Arc 3 could end with them switching allegiances. And Arc 4 could deal with the eventual downfall/everything that goes wrong as a result of their decision in arc 3.
8. Once I have this high level outline, I'd then move to a chapter-by-chapter outlining process, filling in the granular details that were missing from point 7. This includes things like deciding on, "where" a scene should be set, and making sure that no two scenes repeat each other. The "where" is important, because if you stage a scene well, you can use the environment around the characters to tell a story within the story. For example, if you set a scene in a character's house, you can use the prop placements as a reflection of that character's psychology. The lack of repetition is important because it prevents the story from feeling "samey" over time. If an escape from authorities/"flight" scene happens once, it is not allowed to be repeated, unless something new is developed through the repetition. The ideal standard for allegorical narratives is where every chapter reflects a different facet of whatever it is you are trying to comment on, to the point where you could (if you wanted to) name chapters after the things they are metaphors for. This is hard to do right, and in reality, there will be chapters that don't meet this standard, but in an ideal world, this is what I aim for.
9. Actually start writing out the story. This is usually very quick, because at this point, I know what I am trying to do. An important note (at least in my opinion) for writing narratives where the message is the point is that the message always has to be in subtext. If, at any point, you have the characters turn and address the camera and give an opinion on the subject matter (or even if the narrative itself does this), you have failed at writing your allegory. Discussions around the problems within the story should be primarily be consequence focused rather than cause focused, like how doctors often make decisions based around the symptoms they see, rather than around actually knowing for certain what the problem actually is. In a well-written allegory, what the problem actually is, is for the readers to decide.