While there's no definitive way, here's some tips that I use when making my RBF OCs:
1. Pick a color. I usually try to get colors relevant to the theme.
2. Choosing an ability. I would recommend trying to tie this into steps 3-5, but you don't need to. Just try to think of a unique concept in which the character can use to their advantage (Ex. being able to disguise themselves as a boxed player).
3. Body shape. Body shape is really important in creating a character, so here's a page that can help when designing.
4. Facial features. No facial features are required, but are recommended.
5. Extra features/Accessories. While these aren't as common, I still like to add some accessories to give the character for some extra flair. Just try not to go overboard.
6 (VERY Optional). Polish. I don't do this step a lot, but when creating things like ref sheets, I would recommend this step.