Long posts rely on what is basically the essay format you learned in high school, following the old rule-of-three.

Three main sections:

• Introduction
• Thesis
• Conclusion.

Each section is further split into three:

• The basic idea, background, why it matters.
• Three supporting arguments, from different angles
• Thesis restated, arguments summarised, you should agree.

And each supported argument is further divided into P1, P2, C - either modus ponens or modus tollens.

Modus ponens is 'X is true, X implies Y, therefore Y is true'.

Modus tollens is 'X implies Y, but Y is false, therefore X is also false'

Of course, not every long post is necessarily an attempt to convince someone, so you modify the technique to suit the content. Sometimes you're just setting out to explain or inform - but this changes less than you'd think: instead of frogmarching someone towards your conclusion, you're leading them towards understanding. In either case, you still break up the concepts into about three pieces, and present them in an order that makes the conclusion feel inevitable.

If you want to expand beyond that, you can break it down inwards, splitting supporting concepts in three, or you can build it outwards, making three supporting arguments for each basic angle.

One important thing to remember is that nobody wants to read a huge unbroken wall of text, so use paragraphs to break up separate ideas into small manageable chunks with whitespace in between. And remember that the last sentence of a paragraph hits like a mic drop, so use this strategically.

Another trick is to sound out the post in your head and think about cadence; you don't want a string of five-word sentences that all fall off at the end. If you have a whole page of "Dada da da da DUM. Dada dada da DUM. Da dada da daDUM.", your readers will get annoyed and dismiss you without necessarily knowing why. You need to change up the rhythm, throw in some parenthetical clauses, vary the length and keep the flow of tex sounding interesting. It makes the difference between school assembly anouncements and a professional youtuber.

Honestly it's all a bit of a hack - once you get the hang of it, you can hammer it out all day with surprisingly little effort.