So I heard you guys want to write about OCs

This is a copy-pasted answer I gave to a certain user after he/she asked me for advice on how to write an OC adventure story, figured you guys may be interested as well.

The first step is actually to take time and design your OC. Think through personality, backstory and such. Make sure that your character is relatable, this can be done by giving them certain quirks or maybe even handicaps. However, don't overdo it, then you may end up with either an "edgy" character or a so-called "Sympathetic Sue". A good idea can be to base your OC partly on yourself. For example: maybe you're really afraid of the dark. That's a start on creating an OC, maybe he/she could be locked inside an old mine or something, forcing him/her to confront their fear? Once you've established a believable and interesting character (take help from others and toss ideas, otherwise it'll take forever, ask for opinions), you should think about appearance. You can also do these two steps in reverse order, but it's important to note that the second step needs to compliment the first step (either basing the personality and such on the appearance or vice versa)

Here it is important with a few simple things if you ask me:
#1 race (obviously you want to pic one, preferably not alicorn. If you pick alicorn, be EXTREMELY careful with powers, background and the likes)
#2 looks (don't want to look too much like a recoloured canon pony)
#3 palette (use webpages like ColorHexa or Paletton to find complementary colours, just make sure to decide upon a base colour)
#4 Accessories (a bracelet, an earring, a vest. You know, that little flair of... Extra something)

But let's be real, you want to start writing, right?

When you start writing, there is a thing to note.

how does one making an interesting OC adventure story without messing anything up?

See this sentence?

Always remember this: You will mess something up. Everyone does it. It is bound to happen, because you're not perfect. Nobody is.

Capice?

When writing a story, you need to expect criticism AND flak. Both of those will probably strike you. The difference is that criticism helps you grow as a writer by giving you information on what you did wrong and how to improve. Flak on the other hand rarely helps much to improve, rather it, just like actual flak, tries to shoot down your story. Flak is never constructive, and should be ignored.

You probably already know this though, so why am I telling you this?

Well, because OC stories naturally tend to attract more flak than non-OC stories.

OC stories aren't too different from non-OC stories, the major difference being that you don't have an established character whom practically every reader is bound to know beforehand. Thus, it is important that the author get to know your character better than say, Twilight Sparkle (because most of your readers here will most likely have a pretty good idea about who Twilight Sparkle is). This is done by placing the character in situations where one (or more) of the character's trait(s) are revealed/explained. It could be something as simple as seeing themself in a mirror and noticing the dark rings under their emerald eyes, indicating how tired they actually were.

See what I just did? I described the character's eyes without interrupting the flow of the narration.

However, this may not be spread out too much, as the reader may lose interest and have troubles keeping up with who your character is, in other words: don't be conservative with revealing your OCs dirty little secrets.

However, it mustn't happen too quickly, because that's known as infodumping. Infodumping is lazy, unprofessional, and it is more often than not a good way to make the reader nauseous.

Remember to incorporate the information into the narrative, rather than just dedicating an entire paragraph to describing the looks of your kewl OC "Darkwing Maremagnet".

As a general rule, the looks should be revealed quite early, followed by personality and finally backstory. At least if you ask me.

When it comes to the actual story, it is good if the OC protagonist has a flaw of some sort. This is because you want character development to happen, as it makes your character more interesting, alive and relatable. This can be everything from not knowing who his/her parents are to unanswered love, depending on the story.

A good thing to do is to combine this flaw and the main obstacle. For example: have the mare of the protagonist's dreams be the sex-slave of some cruel millionaire whose crimes the protagonist needs to prove in court, have the information about his parents being kept by the tyrannical government forcing him to become a criminal to reach them, have his only way of being accepted by his kind require him to kill a Windigo.

The keyword when writing an adventure story featuring an OC protagonist is character development. Have the character evolve as the story progresses, have the reader empathise with him when he's down, have them get angry at him when he does something he knows is wrong, have them smile with him when he succeeds with a difficult task.

In general, it isn't too different from a regular adventure story, you just need to put more time and effort into making a good character whom the reader can identify with.

The reason good OC stories are often overlooked is because of the stigma created by the vast amount if poorly designed OCs. Many people simply associate OCs with bad stories.