Girl here, and I'm just going of off what's worked for me in the past...

Just relax and flirt when you're around him. Make excuses to be around him when the two of you are alone. Also give him plenty of hugs and physical contact. Do what feels natural, though, don't force things. (Be sure to insist on hugs when you're saying goodbye, though.) Pay attention to his reactions; if he's like "meh, whatever" when you hug him, you're probably out of luck. If he starts initiating physical contact too, you get to be very happy.

If the time is right, and you feel comfortable with it, give him a kiss. Or just wait for him... (I always get caught in this situation where I really want to kiss him, but I'm too shy to do it myself orz.)

After you've kissed, be sure to define what exactly your relationship is very quickly. Friends with benefits situations suck.

Good luck.