1. Get basic idea
2. Write that basic idea down
3. Get a mind map app
4. Make as many branches about character as you can
5. Try to make them by having a thing, and what that thing causes or where it might have come from.

For example, did that while helping my friend with their new oc:
They can see themselves die (not through visions but actually die, only to be actually “fine”), they are a kid, seeing themselves constantly mutilated as a child put a strain on his emotional state and numbness to separate themselves from stress. This has resulted in somewhat “autistic” behaviour, not being able to empathise with other people.
Autism? Why not use then the attachment to animals trait? So they have a teddy bear. What about his parents to support him? Let’s make them put them in an orphanage. That lead to attachment issues and lack of any proper connections. What was then there for him? Let’s make him have connection to teddy and the stars as they were always there. Stars? Why not then give him star pin on outfit.

As you can see you can keep thing thing rolling quite well. Map out ideas, branch out, connect. Have fun!

Ps the character is by @jasenddy so wish her happy bday while at it lmao