I flirt. I usually know I'm flirting, although sometimes the other person reads that I am when I'm not. Actually, it's kind of been a problem in the past...sometimes I don't know when to stop. If someone flirts with me, it's almost an natural impulse to flirt back.
It's a bit of a science. You can flirt in banter; there are some guys that really like to be challenged. You can flirt directly; that's something you do more with people you know well. You can flirt just by showing a sort of open interest.
BUT it's all in the body.
If you're flirting with banter, you should be either leaning forward in sort of a bored, interested way, or leaning back in an bored, open way. And you should allow yourself to smirk, raise your eyebrows questioningly, etc. Keep your expression, especially your eyes, playful. That says, "I'm comfortable, I'm interested, but I'm feisty and want to challenge your manliness." This goes well with T guys and guys that like to show off...tested against ExTPs for sure
If you're flirting directly...well, you flirt. You can allow some touches, some openness, but no so much that it's stifling or desperate. I usually only do this as a joke with close friends, or with established love interests.
Open interest is possibly the easiest for INFJ. Just show that you're sincerely interested in what they're saying. Some shyness is even a little cute. Smiles are good, and show the natural body language for interest: lean forward but stay loose and natural. This is more allowing them to flirt with you
This works better with NF guys or introverted F guys...or guys with a sexist complex, if you go for that. It's less domineering, and allows for more opening up and intimacy