What are ways one can make a self-insert work?

Write them like any other character. Have the self-insert follow the same rules of the universe as them. Don't show the self-insert special treatment just because it's "you" in there, lol. In fact, it's probably a bad idea if you tell your readers that it's "you" in there. Let them guess. That'll make it more fun.

As a blatant wish fulfillment example on what NOT to do, let's say your self-insert takes Ash's place in one of the anime seasons, still having Ash's companions (but Ash himself is nowhere to be seen). Let's say the self-insert is actually a "newbie," but the self-insert does everything that Ash ever did better. In battles, decision-making, etc.

Maybe the self-insert even gets preachy to Ash's companions about how to be a better trainer, and then they just simply agreed to it without argument (or there IS an argument, but they were proven "wrong" later). One of them becomes the self-insert's lover too. All conflicts get resolved easily, meaning the rival sucks, Team Rocket is never an issue and the local regional evil organization is full of incompetent mooks. The self-insert may acquire some legendaries later on, enough to the point to make Tobias from the anime seem less excessive. And win the local league easily.

Congratulations, Champion Mary Sue! The people have chosen you to the be the Emperor/Empress of the world! All of the Elite Fours and the Champions in other regions bow to your incandescent glory!

And there you have it. Just write the self-insert as a normal character. Put YOURSELF in the shoes of your other characters. What would YOU realistically do during certain situations? Would YOU make different decisions in their stead? You can probably get away with making the self-insert more ideal (your own personal aspirations) from time to time, but take care not overindulge. Maybe you had bad acne in real life; but in the story, your skin is clear. And that's fine. Maybe you weren't that strong or courageous in real life either; but in the story, you gain both from character development. That's fine too.

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