There is no fixed rule for writing a romantic love letter as it automatically comes from within. You can write the way you want to and say what you think is heartfelt. The key ingredient for doing the same is to be true and tell what he/she means to you.
Make sure that you tell her for the way she has changed your life and has given you a reason to live and love your life, the reason why you look forward for having a life together. This is the time when you can make her feel special by expressing what she means to you and how important it is for you that she stands by your side in every decision and for life. You will have to admit at this time that it is important for you to say all the nice things that you ever wanted to say