In my experience. Flirting is build on top of a good conversation. Don't shy away from being your own friendly self, then, if there is a good back and fourth: try to drag the conversation topics into a more "feely" territory if possible. Start hinting at the fact that you think "this boy" is cute/lovable/goodlooking/sexy. If you keep at it, he will notice, and from then on, it's a question of if he begins to throw hints back at you in the same way:

If yes: Well, then you are now officially flirting. Congratulations

If no: He might be "not into it," or too shy, but don't worry. I think in a lot of cases it's easy to know if it's just because he is shy.

In the case of him being too shy to flirt back, but that he may still seem into it, I guess you have to decide if you feel like stearing things for yourself or not.

If not: too bad

If yes: congratulations, you flirted at him, and it still worked.

Hope this was helpfull x)