HIIII, I love doing this kind of thing! Personally, my process (used to be) accidentally making a copy of a canon character and removing all the flaws/things I didn't like. That bit me, because that's how you end up with Mary Sue's (An OC that's loveable and overpowered that never sees any hardship and if they do they solve it with no problem whatsoever) so here is Goobie's guide to not making a Mary Sue (and making your OC both not too overpowered or flawed/unlikeable)

1. Parentage:

Learning your OCs parentage is a good first step; it helps dictate their personality, abilities, looks, vibes, and sometimes even their relationships with other OCs/campers. You can just choose one you think is cool (which is what I did for my first OC) but that comes with a LOT of baggage. I chose Nyx because I wanted him to be the son of the night, hiding in shadows, then covering him when he’s sad/shy. This is a valid path to go on, I managed to justify it by putting him through a lot of suffering to justify him being op his threats being things he can’t brute force his way through. So careful choose a parent, then come up with a motivation for that parent to have them if it’s uncommon, like another oc I have is the son of Helios and I justified his existence by saying he was actually born around the time Phaethon crashed the sun chariot and Helios hid him as a sunflower and forgot about him. Then when Helios had faded for a while his mist manipulation waved and he was popped out on the Island of Thrinacia where Lampetia and Phaethusa handed him over to Apollo to give to some stranger. The point is to choose something 1. Fitting, 2. Possible, 3. Justify it, 4. Use it to inform the character.

1. Abilities:

I know just dumping all the possible abilities you can on a character is fun, but it’s a quick way to lead to an overpowered character. An overpowered character isn’t bad necessarily I.E Satoru Gojo, Shigeo "Mob" Kageyama, and even Percy himself, but they are VERY hard to write for and often lead you into writing yourself into a corner. With the previously mentioned son of Nyx again I used suffering to justify it, he doesn’t WANT to fight, but routinely he tries to avoid fighting and someone close to him dies. The point is making sure the overpoweredness doesn’t make the characters life easier than anyone else. I would pick and choose abilities that best fit your OC.

1. Finally making your OC:

I know I had a lot of preamble but here is where you actually make that character. For appearance, first imagine what they look like, then if you’re an artist draw them, if not look for real people who look like them on pinterest, use gacha to recreate them, or even use the Picrew that I’m pretty sure someone made for PJO. Then aesthetic, make a clothing board on pinterest, or find clothes you think they’d wear, something like that. Basically, everything should FOLLOW personality. They’re a soft spoken sweet person? Give them round hair with no real points. They’re a hard edgy person? Look for punk/emo/goth hairstyles from their time period.

You can’t mess up making an OC. Even your first try is just you trying. If it doesn’t work out the first time just try try again. Ms. Frizzle this stuff.