Simply say "I love you."
Of course, it's still a much harder thing to do than that.
When I first gave the hint to my now wife, we were in a little gift shop in Branson, MO. I found a tiny wooden box, barely bigger than my fingertip. When I opened it up it had a wiggly ladybug inside of it, and on the top of the box inside it said, "I (picture of a heart) you." I took it over to her and opened the box for her. She looked at me and asked, "Are you trying to tell me something?" I said back simply, "Maybe," and smiled.
One caveat here which I think is vitally important is that when you tell her you love her, you truly do. How do you know when you are in love with someone? When you can't remember when you weren't.