There are many ways to express your love. If you are lucky, she also likes you. Then any way of expressing your love is very romantic. Here are some suggestions for you. I am a girl, so I should know more about girls' thoughts.
You can place red candles in a meaningful place in the shape of a heart, light them, take her there, tell her what's in your heart, and give her a bouquet of roses. I think this can impress girls.
Have a candlelight dinner together, then watch the night view together, and then give her a glowing pillow, which symbolizes the twinkling stars in the night sky and accompanies her all the time. It's quite romantic.
You must know each other well, and you must know what type of girl she is. Then you can cater to her. If she is a girl who loves reading, you can give her her favorite book and add your confession in it. This method is very safe. Even if she doesn't feel anything for you, at least you won't be embarrassed to be rejected when you confess to her in person.