Okay so this is obvious. But the first step the flirting, is to have a very good crush. Like, an intense crush. Like a "I WANT TO KISS THEM LIPS MHMMM!" Crush. You also need to know the person a bit...like maybe become friends with them. Do NOT. DEVOLP A CRUSH, ON A FAMILY MEMBER. THATS NOT OKAY. You also need to know that you FOR SURE have a crush on this person. I'd suggest waiting a bit to make sure that you really do have a crush on them. For example: I've had a crush on Tom Holland for over a year now, so if I could (and if I was old enough) I'd surely flirt.
(UPDATE: I now have a crush of Bakugo from MHA, so if I was 15 I would definitely flirt with that angry Pomeranian)
(ANOTHER UPDATE: I now have a huge massive simp crush on Tenya Iida. I want him to pin me against the wall and make out with me on the spot rn.)