Let's say you don't know how to flirt. I shall teach you What NOT to do.
Don't be a jerk when you flirt, because that's just rude. Teasing, pushing, laughing at (in a rude way) your crush may turn them away from you.
Don't just assume that you're flirting is working. Just because you're saying fancy words along with nice ones doesn't mean it's working.
Don't force them to love you with your flirting, that's not right and could lead to them having trust issues with you.
Try not to make them uncomfortable. If they seem to be becoming uncomfortable, back down and stop flirting, maybe change the topic or something.
ABSOLUTELY DO NOT TRY TO KISS THEM WHILE YOU FLIRT (unless you two are a couple)!! That may make them EXTREMELY UNCOMFORTABLE and they may not want to be around you anymore.