To make a girl blush, I find that genuine compliments go a long way. It's all about being sincere and specific. Instead of just saying someone is pretty, tell her that her smile brightens your day. Little surprises, like leaving a sweet note or making her favorite drink, can also do the trick.
Being playful with teasing can create a fun vibe, too, but always keep it light and respectful. Lastly, maintaining eye contact while talking can create an intimate atmosphere that might just make her cheeks turn red.