Prequels are Probably a Bad Idea
I understand why you might want to write a prequel. You enjoy your characters. People ask about how they got to be who they were when the story began. You have some fun world-building that you'd like to share. You want to tell the history of what occurred before your earlier works.
But I want you to ask yourself... what was the last good prequel you watched? When was the last time a story was enhanced by flashing back to some characters from before their first story?
If I've just seen how the story ended, what am I going to get out of going back and seeing how it started? And if I watch the prequel first, what does it actually add to the experience?
What is the Ideal Viewing/Reading Order
I’ve yet to encounter a prequel that actually improves on the story by being watched first, with one slight exception - Revenge of the Sith before Return of the Jedi. And even then, that only works in the Machete Cut.
For those unfamiliar, the Machete Cut seeks to solve a problem that arises when you introduce new people to Star Wars. Do you start with the original trilogy and then do the prequels? If you do, the prequels hold very few surprises. Do you start with the prequels and then move to the original trilogy? The problems there are worse - the prequels don't work as well on their own, because they build off of what was set up in the original trilogy. (They also aren't very good.)
The Machete Cut solves that problem pretty neatly. It proposes that you watch A New Hope, and then The Empire Strikes Back, and then you watch the prequels before moving on to Return of the Jedi. This actually fulfills a few, very helpful functions for the audience:
After the reveal that Luke is Vader's son, it gives us a chance to see how Vader's father fell from grace, and introduces new dramatic tension around it, because we don't know how Vader's story ultimately ends.
We see the story play out, and then we get Luke confronting Obi-Wan about it in Jedi, which actually gives us a chance to choose a side. Do we agree with Luke that Obi-Wan distorted the events, or do we understand Obi-Wan was carefully talking about a sore subject that still weighs on his soul?
It puts three movies between the Luke and Leia kiss and the reveal that they are siblings.
It actually strengthens the reveal about them being siblings. In Return of the Jedi, it feels tacked onto a scene where Luke talks to Obi-Wan about Vader. It ultimately has very little impact on the story. But if you're watching Revenge of the Sith first, you watch Luke's mother go into labor, and then find out she's carrying twins. Then after Luke is born, his sister is delivered... and Padmé calls her Leia. And what felt like checking-off-the-boxes when watching Sith last, now feels like a reveal.
But why does this work? Because it turns the prequels into a much stronger narrative device - a flashback. And a flashback that comes before the finale can actually inform a story and make it stronger. (Granted, it would have been stronger if it had been written that way, and also if the films had been good, but that's neither here nor there.)
Usually, the argument against prequels says, "I already know which characters are going to live and die, so what is the dramatic tension?" I'm not going to insult your intelligence: there are lots of good movies where we can reasonably assume the main character isn't going to die. Just about every action movie hero will live, and we know that when we buy the ticket - we would be awfully surprised if Indiana Jones died in only his second film.
But dramatic tension doesn't only come from character death. So, ask yourself: what does this add to the character's arc from their first appearance?
Let's look at two bad prequels, both for Harrison Ford characters: Indiana Jones and the Temple of Doom, and Solo. Because it turns out, both Han Solo and Indiana Jones were introduced at the perfect point in their journeys.
Solo proposes that Han wasn't actually all that mercenary - he was a good guy with a heart of gold, who helped people in need. So, that's a nice story about the version of Han that we have for most of the trilogy... but not a very interesting story about who he is before A New Hope begins. If he is a good guy who helps people in need... then why is there any question about how he'll behave in A New Hope? Why is it a challenge to recruit him to rescue the princess or join the Rebellion?
It's okay if Han Solo changes his perspective and grows more cynical between his solo (ugh) adventures and the original trilogy - people can change. (A similar character, Malcolm Reynolds, grows more desperate and mercenary between the Firefly TV show and the sequel film Serenity, and it makes perfect sense.) But if your goal with Solo is to explain his origin and how he became the character we first met... they missed.
Temple of Doom somehow misses the mark almost as bad, despite being made by the original film's creators, in between two really good installments in the same franchise. In Raiders of the Lost Ark, Indiana claims he doesn't believe in magic, and at the end he surrenders to faith and trusts in the Power of the Ark to kill the Nazis. It's a moment of growth for the character.
But at the end of Temple of Doom, Indiana prays to Shiva and the magic rocks burn the bad guy and make him die. And this film, for some reason, takes place before Raiders of the Lost Ark. It makes no sense, and it's bad and I hate it. If you're undercutting your main character's entire arc from the first film, you've officially made a bad prequel.
Closing the Loop
Your audience will likely ask a lot of questions about the backstory of your world. Some of these are deliberate questions you set up in your writing to add intrigue, and others may be elements you didn't consider unresolved or mysterious. But eventually, people will ask enough that you'll consider flashing back and showing how things happened.
If you do write a prequel about history referenced in the original story, ask yourself: Is that literally all I'm doing? If there's nothing else you're adding, no other elements to make this a self-sufficient story in its own right, then stop typing right now.
What is This For?
Too many prequels rely on the concept of "being a prequel" and forget that, if we know the ending, we need a new dramatic tension. Are you telling the story about a war that shaped the world of your first story? Then the ending can't be the most interesting part of your story, because anyone picking up your story probably already knows the ending. You need something else to hook them in.
This is actually where I recommend you go watch the movie Titanic again. We all know the Titanic is going to sink, and on the off-chance you don't know the details, the film doesn't jerk you around - it shows you the wreck right at the beginning, and tells us the textbook-style version of events. And then the story flashes back (See? It's a much better dramatic technique than a prequel), and we meet the main characters we'll be following through the story. The question of "What will happen to the Titanic" doesn't matter - it's answered during the prologue. Instead, the story asks, "What will happen to Rose and Jack?"
So, if you're doing a Game of Thrones prequel TV show about a centuries-old war that formed the Seven Kingdoms, make me care about the characters, because I sure as heck don't care about the outcome of the war.
Disclaimer: If I end up writing a prequel some day, you may be tempted to go back and see this article as some sort of sign of hypocrisy. Can't help that. Hopefully this hypothetical prequel is good.