(woop sorry this is long)
well, when I create a character first I start with the character's story. I find that if I design the character beforehand, it limits the background of the character. Make sure it's not obnoxiously OP, and make sure they have some flaws. ex: arrogant, hypocritical, over-competitive, dishonest... I find that flaws that real people have made the character more real. Flaws make the character more interesting. Figure out their past. What did they do in their past that shapes them today? What about goals? Do they have a goal they are reaching for? One thing you have to make sure you don't do is reject the normal. I've seen all too many characters that parents died in a fire... they are bullied.. and they lose the love of their life.. that kind of thing. Its ok to add one or two mundane things into your character's story that will draw interest, but make sure there is a balance. Make sure they have down to earth traits as well as cool traits. If you have no idea where to start with a background I recommend getting inspiration from a background generator(of course don't just let the generator decide everything) but maybe it will give you some small ideas.
Once you have your background now you can design what your character looks like. This part is very important as to not making your character hurt peoples eyes when they stare at it. You probably decided your character's race and gender from when you were writing their background. My advice: don't make an alicorn oc. The hardest thing to do is to make a likable alicorn oc. My advice is to just avoid it. First thing I like to do is pick a color scheme. Now, when you pick a scheme, harsh colors are a no-no. If you don't know a thing about color theory and have no idea where to start, look up color schemes on google or find a color scheme generator. Your scheme can somewhat link to your character's background, Ex: Luna's color scheme matches the night. AppleJack's color scheme has yellows and oranges- colors of hay and reminds me of farms. But it doesn't have to. Look up images of hair on google and try sketching out your character with ones you like on them. Even if your not an artist its good to get somewhat of a visual of what they look like until you find someone to draw your character for you- stay away from the pony creator game haha. Look up some real horses and look at their fur patterns if you want your character to have patterns. There are many different options for your character and just mix and match and sketch things out until your happy with what you see. The most important part to this is to have fun while you are doing it and to be proud afterward.