How do you flirt with a guy so he knows you’re interested?

I never get sex even though I meet loads of guys, starting to worry I feel invisible to guys why don’t they see me I that way. What can I do to signal sexual interest and attract men?

I feel my geeky slightly unattractive male friends (I love them though they’re gre at) have more success than me with sex, guys just don’t pursue me for anything and it’s so lonely