Yes, eye contact, but you need to enhance that with the tilt of your face. You look from beneath a slightly lowered brow. As if shy. Your face is a live, smiling and reacting, pouting (fake pouting), and doing whatever to show you are really interested in this guy. Your reactions have to not go over the top, but they should be as strong as you can make them (positively, usually, although surprise can be big) without making them fake and hammy.
You can make this easy by following your feelings. You shouldn’t actually have to try to flirt. You just let yourself show your feelings. Real feelings. If you like the guy and you don’t hold your feelings back, you’ll probably flirt for real (but not like a pro—that’s too fake). Try to not pressure yourself. Just focus on your guy and let what happens happen.