Before you ever think of saying it, you have to build a foundation of friendship and trust. Once you're ready, be sure you have no distractions, look her in the eyes, and tell her that you can't live without her. Or that you are the luckiest guy alive just for having known her. As long as you're sincere, it won't matter what words you use.
You will have to say it verbally but your actions should speak more of how serious you are. This is because you cannot say you are serious if you are not there for her and also not understanding when she needs your help. A girl will know if a guy is serious if he is ready to meet her parents.