I'm new to RPing and Grace Lesley Kettlewell was my very first character, and I know I sucked at her. So I'll talk about how I created my second and much better carrie: Evan Cartwright.
I've read some people's advices, and most of them start with personality. Well, I always start with background. Did they have any tragedy? Or is their family rich and likes to make their children spoilt? Where do they live? What did they do?
Then I let the background create the personality. Were they raised with much money, so they're selfish, arrogant, mean and spoilt? Or did they have no money at all and therefore knows to share, but also exremely jealous? Then I write their interests and dislikes, again sticking to their background. I always add something from their house to the personality. For example, Evan suspects of Muggleborns and hates Muggles now.
Then comes their appearance, and name. For surnames, I always check my favourite series&books and pick one.
I had completed Grace's personality, so I didn't enjoy RPing her and dropped. But I didn't complete Evan, and now I really like him

The only flaw Grace had was her allergy, but Evan has much many. So yeah, leave some flaws and remember: your RP will improve your character

And if you're unsure how to create the personality, you can write yourself

Evan has a lot in common with me.